To evaluate the spatial resolution performance of PET systems, Derenzo published an analytic resolution formula in 1993 to analytically calculate spatial resolution in the center of FOV which accounts for impacts of geometrical, instrumental, reconstruction and physical factors and is now widely used to guide PET system design. Extending from this study, the crystal size is replaced by FWHM of coincidence response function, and then a modified approximate spatial resolution formula for circular configured DOI-PET is presented. This formula can account for the DOI resolution and estimate the overall system resolution over the entire FOV. With this formula, one can also predict uniform resolution condition and best achievable resolution. Monte Carlo simulations were performed to test the formula. Formula agreed well with experimental results.