In this paper, Impulse Response Function in Circular Synthetic Aperture Radar Imaging (IRF-CSAR), which is a special version of Impulse Response Function in Ultrawideband-Ultrawidebeam Synthetic Aperture Radar Imaging (IRF-USAR), is presented and shown to be valid for representing the CSAR image of a point-like scatterer. IRF-CSAR can therefore be used in studying different CSAR systems such as predicting the pattern of a point-like scatterer illuminated by a CSAR system, estimating resolution achieved by that system. Applying IRF-CSAR to define the image quality assessments for CSAR is also presented in the paper. (6 pages)