Abstract: A method based on the Compton scatter theory is derived to compensate the effects of single and double scattering in Tl-201 SPECT.The scatter medium is divided into small volume elements and the scatter counts contributed by such a volume element are obtained by integrating the number of photons scattered by all electrons within the element.The scatter probability from an electron is determined by the Klein-Nishina formula.The counts are then assigned to the detector bin that is the projection point of the center of the element.In this way the singularity in calculating the scatter counts near a point source is eliminated.The main approximation is the finite volume of the element and the limited range of scatter medium included in the calculation.In spite of the approximation,the calculated scatter line source response function and scatter-to-primary ratio agree reasonably with those from Monte Carlo simulation and the computation time is shortened 40 times.