The effect of stress during annealing induced by Si3N4 films on boron diffusion in float zone-silicon has been studied and a correlation between vacancy concentration and compressive strain in the substrate has been clarified. From the results of Si3N4 film thickness and annealing temperature dependence on both boron diffusivity and stress in the substrates, boron diffusion was found to be retarded and the substrate was found to have high stress, having Elastic compressive strain during annealing under Si3N4 films. These results indicate that excess vacancies are generated by elastic compressive strain, causing the retardation of interstitial-mediated diffusion of boron.