Is it possible to make circle (curved) blocks in minecraft?

If you want bloody perfect circles larger then 1 block, use invisible minecarts with blocks on them, arranged in a circle. It's hard to explain, and uses tons of command blocks, so I'd recommend watching this video

But this would take up lots of time and space, so instead, you can do it the non-command-block way:

Dig a hole 1 block wide and 2 deep. Then take a mob spawn egg (I recommend chicken) and right-clock into the hole. After a minute or two, stop, and look inside. You got yourself a perfect circle in the middle of the hole.

There's also Architecture Craft, which adds circles (and arches) to the game. It may be helpful.

Here is the link. http://www.9minecraft.net/architecturecraft-mod/