Hi emrah_ilhan,

GEC Member sgrillo has a very nice freeware utility called GE Path which allows you to create circles (and other shapes) with specified center coordinates and radii lengths. The program then creates the kml files for you.

http://www.sgrillo.net/googleearth/

If that doesn't meet your needs, try asking this question in the KML Developers Group:
http://groups.google.com/group/kml-support?lnk=li

L.