I try to perform a fillet b/w closed circle and line, it is a 2d sketch or drawing of catia..but it doesn't allow same in solidworks why ?
Its quite easy to use a fillet in solid works. You just have to pick two sides in which the fillet have to placed and then choose the option of fillet. It would be easily done. For circle and a line you would probably be doing something wrong so try using the rebuild feature and there are yet another options available as concentric and tangent that can be applied. Be more specific so that it would be helpful for us to understand your problem and answer it.