The profile of the point spread function (PSF) in superresolution microscopy is studied theoretically. The fluorescence spot profile (i.e., the PSF) is determined by the focused beam patterns of the applied two-color lasers and the optical properties of the fluorescence-depletion process induced by the lasers (the pump and erase beams). In this study, the fluorescence-depletion process for the sample molecule is analyzed using a rate equation for a three-state model. Based on this result, we calculate the PSF for the case where the erase beam is modeled by a first-order Bessel function. In the case of an erase beam with a large photon flux, the obtained PSF has a Lorentzian-like shape, which seldom appears in traditional microscopy. In this work, we also investigated a possible relationship between the PSF and other parameters in the fluorescence-depletion process. © 2005 Society of Photo-Optical Instrumentation Engineers.