QGIS radius buffer question, can it be more circular?

The "segments to approximate" option is what you are looking for. So the number you put into that filed will be the number of sides/quarter. So the default is 5, so you end up with a 20 sided polygon. Put in 25, and you end up with a 100 sided on, so it becomes smoother.

I don't know how to make "perfect" circle buffer with qgis.

But, the "Segment option" at "Fixed distance buffer" means:

"How many segments do you want to make in 90 (360/4) degrees?"

If you got 5 segments, you will get 20 = 5*4 segments totally (in circle) and that segments will have 90/5 = 18 degrees each.

If you got 9 segments, you will get 36 = 9*4 segments totally (in circle) and that segments will have 90/9 = 10 degrees each.

So, the larger number of segments, the smoother the circle will be created.

You can control the resulting number of segments in the Buffer dialog box.

And, if you want more, search for "buffer" in the Processing Toolbox:

You can use all of these algorithms (from Ftools, GRASS GIS or SAGA)