Improper forest management frequently causes serious damage to hill slopes and torrents. This study examined the influence of annual variation of deforested and reforested areas on slope failure in Miyakawa-dam basin, Mie prefecture and Nagasawa-dam basin, Kochi prefecture. The followings were observed in this study: 1) Collapsed area is proportional to the non-forested area and inversely proportional to the reforested area. 2) Slope failure frequently occurred after 5-7 years of deforestation in both the basins. In reforested sites of Miyakawa-dam basin, slope failure declined after 20 years of reforestation. On the other hand, many slope failures occurred in non-forested sites even after 24 years of deforestation. In Nagasawa-dam basin, there occurred little slope failures after 10 years of deforestation. Using these findings, a slope failure prediction model considering precipitation and forest management conditions was developed, and the model was applied to Miyakawa-dam basin and Nagasawa-dam basin over a 30 years simulation period. The comparison of the measured and simulated collapsed area ratio showed that the model was adequate for predicting the slope failure areas of both the basins.