Molecular dynamics simulation was carried out to clarify pressure effects on diffusion constants of point defects such as a vacancy and an interstitial atom under constant pressure by using Stillinger-Weber potential. The calculated results indicate that the pressure effect on diffusion of the point defects is small during single crystal growth of silicon, since stress, which was obtained by a global heat and mass transfer model is not enough to modify migration process of the point defects. Activation energy of a vacancy and an interstitial atom was obtained as a function of external pressure.