Abstract: Fluorescence dispersion of Lu2SiO5 crystal as a -image converter was analyzed in theory and point spread function (PSF) of LSO scintillator was studied using Monte Carlo method. The simulation was divided into two processes. The energy deposition distribution of -ray was calculated with MCNP code and transferred to distribution of light photons in the scintillator. PSF was obtained through the Monte Carlo simulation of light transport, which was not available by MCNP code. The result was verified by PSF measurement in the knife-edge experiment. The FWTM (full width at tenth maximum) of PSF, which indicates spatial resolution, was calculated as a function of incident -ray energy and LSO thickness.