Light absorption and scatteing coefficients of phosphor grain with different sizes and packing densities are computed with Mie theory.Optical photons in a granular phosphor screen are modeled by simulating scattering and absorption events of individual optical photons by Monte Carlo method (MC).Depth-dependent optical collection efficiencies are obtained.Using MCNP,distribution of energy deposition of 6 MeV mono-energetic X-ray in tungsten and phosphor screen is modeled.The point spread function of screen is given as a convolution of energy deposition kernel and optical kernel.Granular screens of smaller grain and higher packing density exhibit better spatial resolution in megavoltage radiography.