Abstract: The pixel values of a calibrated radar image are quantified in terms of the same unit of radar cross section (RCS). However, it has long been an argument that should the pixel value be considered to represent the practical RCS level of a target or not. In this work, two concepts are first introduced, namely, the target scattering function and the target scattering distribution function, which are defined with coincidence to that of conventional RCS. Combining the concepts with an analysis of electromagnetic scattering mechanism and radar images of a classic target, the understanding of high resolution radar images as well as their pixel values for complex targets are discussed. It is concluded that, the pixel values of a high resolution radar image should not be simply explained as the RCS level of a target, but Parseval's theorem holds between the data in spatial frequency and in image domains. In the cases when with a small imaging aperture angle, the average RCS in spatial frequency domain is equal to the summation of the pixel values of a magnitude image.