Theoretical investigation is presented to analyze and optimize the performance of passively Q-switched Nd:GdVO4 laser with Cr4.:YAG as a solid state saturable absorber. A simple model is developed based on numerically solving the passive Q-switching rate equations, where the impact of the saturable absorber excited states photon absorption is included. The effect of both the saturable absorber doping concentration and output coupler reflectivity on the output laser characteristics are studied. The simulation results show reasonable agreement with those obtained experimentally by other research groups.