Abstarct: In diagnostic nuclear medicine different factors affecte image quality, for example collimator, septal penetration of collimator, structure of scintillation detector, electronic limitation and etc. One way to evaluate these factors is using Monte Carlo simulation. So in this research the role of collimator (Geometry and dimension) on spatial resolution of gamma camera by using MCNP-4C is studied. In order to investigate the effect of geometry and hole diameter on spatial resolution these collimators: cylindrical, cubic, hexagonal, converging and diverging pinhole, multi-pinhole cylindrical and multi-pinhole cubic collimator are simulated. The results show that pinhole collimator like hexagonal, cubic and cylindrical that have approximately same geometry, have same process in changing spatial resolution by increasing hole diameter, also converging pinhole collimator has higher resolution than diverging pinhole collimator. In multi-pinhole collimator except in one millimeter diameter, cylindrical collimator has high resolution while in other hole diameters, both multi-pinhole collimators have same process in spatial resolution changes. The conclusions indicate that hole diameter is an effective parameter on spatial resolution for same geometry shapes.