The band transport model is now a widely accepted photorefractive theory. In this model, diffusion mechanism acts as a main role and in some cases two modifications, i. e. , drift under an applied field and the photovoltaic effect, are added. Actually, the photovoltaic effect should aloways be considered as important as the diffusion, and in the case of fcrroelectrics, e. g. , LiNbO3, this effect often dominates over the latter and thus becomes the main photorefractive mechanism. In this paper, the phenomenological theory of photovoltaic effect was developed and an all-around analysis for the formation of the photorefractive grating is given.