Many slope failures have been observed to occur during or immediately after rainfall. Although conditions leading to these failures have been described as caused by a rapid rise in pore-water pressure as a result of rainwater infiltration, the important factors that influence the initiation of slope failures have not been adequately clarified. To investigate these factors, a series of laboratory experiments was conducted on model sandy slopes to determine the initiation process of rainfall-induced slope failure. In the tests, failures were induced in small-scale model slopes either by water percolation from the side upslope or by artificial rain falling on top of the slope. Besides monitoring pore-water pressure, changes in soil moisture contents and ground deformation were measured. Test results showed that slope failure was always induced when the soil moisture content within a certain region near the toe of the slope reached nearly full saturation, even though other parts of the sliding mass were still in a partially saturated state. In addition, minute deformations along the slope were shown to precede failure. The findings presented here show that by monitoring the soil moisture content of slopes and performing displacement measurements, it is possible to predict the occurrence of rainfall-induced slope failure.