Molecular dynamics simulation was carried out to estimate diffusion constants and mechanism of point defects such as a single vacancy and a self-interstitial atom under hydrostatic pressure. The Stillinger-Weber potential [1] was used as a model potential, which is widely accepted for modeling of silicon crystals and melts. We obtained the following results on a self-interstitial atom from the calculation. (1) Diffusion constants of self-interstitial are almost independent of pressure in the range from - 50 to + 50 kbar. (2) A self- interstitial atom diffuses with the formation of dumbbell structure, which is aligned in [110] direction. For single vacancy, the following clarified. (1) Diffusion constants of vacancy are also independent of pressure in the range from - 40 to + 40 kbar. (2) A vacancy diffuses with a switching mechanism to the nearest-neighbor atoms in lattice site.