Ummm It might be that, I use Pyxel Edit since 2016 xD

"Perfectly round" circles aren't possible when you're working with pixels, they'll always be at least a little imperfect. The larger the circle, the easier it is to get it to appear smooth and circular, but whether the longest segment has an odd or even length doesn't matter. Maybe your program's circle tool just has some strange bias that makes weird circles when they're certain widths xP That stuff can all be fixed manually.

Thanks so much for your help! You helped so much that I don't know what to say..

Also, I'm doing a lot of circles, and I noticed, that to make a perfectly round circle, the start needs to be an even number? Is it true? 

Sorry if you can't understand, my English isn't that good 

Hard-refresh (ctrl+F5 in most browsers) and the new images should show up. Your browser is showing the old, cached version. For some reason PJ doesn't tell the browser that the image has been updated and needs to be redownloaded.

It's hard for me to explain what makes things jagged. The basic thing is to keep line segments either consistent or growing/shrinking, don't go short-longer-shorter-longer-shorter when you want a smoothe shape. The other thing with curves is you want to "pace" the increases, don't go "short - slightly longer - much longer" or "short - much longer - slightly longer", go "short - longer - longer".

Here are visual examples.

The top row just shows how mixing segments of different length randomly gives a jagged line. The first two "correct" examples rearrange those same line segments (a 1, a 2, and two 3s) into surves, one curving up, one curving down; these segments look smooth because the segments are arranged so that they either always increase or always decrease in length, they aren't mixed. The third "correct" example is a straight line of approximately the same angle, it looks smooth because all the line segments are the same.

The second row shows the difference between mixing line segments of different lengths, and keeping them increasing/decreasing consistently. The red one looks more like an S curve, it appears to change direction. The green one curves more consistently. This is the problem you have with the uppermost, lowermost, and left/right-most parts of your circles. Because you're mixing line segment lengths, the curve appears to change direction where it shouldn't.

The third row demonstrates another problem found in your circles, jumping between long and short segments with no inbetween. Rather than a smooth curve, this creates a blunt corner.

Here's an annotated comparison of the circles in your version and in mine. I wrote in the line segment lengths, and the difference from the previous segment. I also marked increases in yellow and decreases in purple. See how in my edit, the decreases are all grouped together, and the increses are all grouped together? In yours, they're all mixed, creating the jagged look.

I hope this helps you spot and correct jaggies!

Thanks a lot! I re-did it and re-upload it! (But I still can't see it here)

And also, I want to ask you something, how do I make the shadow more like, round and less jagged? I have a lot of troubles with that.. because as you can see in the new image, I see the shadow a little square..

If you're still working on this, the forum is probably the better place to post.

It looks strange that you go from midtone (the green) to dark (the dark green) to light (the backgrounds). What is the purpose of the intervening dark colour? I'm guessing on the left it's meant to be shadow. If that's the case, I think extending it more so it's clearly more like a shadow rather than a partial outline would help. The dark green on the right part of the images makes no sense at all to me though. They just make everything look blockier, despite your hard work AAing.

In addition, the shadow on the left is a little jagged, and that jaggedness gives the whole piece a less smooth feel, despite the AA.

The AA colours in the bottom left and top right (green background and red background) versions are too light. Aim for an AA colour that's approximately halfway between the values of the colours being blended. That AA colours there have values very similar to the background, so they just blend with the background instead of easing the transition between it and the darker greens of the face.

The colour choices for the AA in the other two are effective.

I think you might have over-AAed a little. You don't need to AA 45-degree lines. These circles have four short 45-degree segments each, and putting AA on them just blurs things instead of making them smooth.

Here are a couple of edits of the bottom right (yellow background) version. The left one has your original circle shape but with a slightly enlarged and less jagged shadow, the dark pixels on the right are removed, and I re-AAed it (with less AA overall!). I think these changes help it look smoother. The version on the right expands the circle a bit so it looks rounder and smoother.