The effect of stress in silicon nitride films on boron diffusion of silicon has been studied. During annealing in nitrogen, the degree of the retardation in boron diffusion becomes larger as the temperature becomes lower and the thickness of nitride films increases. On the other hand, boron diffusion of Si covered with very thin nitride films is enhanced for long diffusion times. These results suggest that nitride film stress makes the region near the surface of the silicon substrate compressed and changes vacancy and interstitial concentrations.