Currently, the conventional empirical or semi-empirical dose calculation algorithms for radiation treatment planning system have been selected as dose engines. The accuracy of these dose calculation algorithms is limited. The main problem is that they fail to adequately consider the lateral transport of radiation or loss of electronic equilibrium near interface between two heterogeneous mediums. Monte Carlo method provides a kind of precise and general solution to photon and electron dose calculation problem, but the puzzle of the longer CPU time affects its clinical application finally. The Monte Carlo code system (EGSnrc) has been used to investigate the different voxel sizes effect on the accuracy of dose distributions and computing time in the present paper. Based on a patient head and neck CT data case studied, it may be concluded that less than 4mm voxels should be used for Monte Carlo dose calculations to insure the superior accuracy of dose distribution, higher image resolution and shorter CPU time.