By using a small flume, a series of tests has been carried out to trigger rainfall-induced-slope failure. The detailed monitoring of excess pore pressure and the soil displacement revealed that a high excess pore pressure was generated not before the failure, but during the subsequent motion after the failure. The variation of shear stress, normal stress and pore pressure during failure shows that approximately full liquefaction failure could be initiated even when the soil was in naturally drained state in slopes. By using silica sand no. 7 (D₅₀ = 0.14 mm) and no.8 (D₅₀ = 0.0576 mm) and changing the samples' initial dry density and flume angle (20 and 30 degrees), the effects of grain size, initial density and flume angle on the generated pore pressure and fail-ure mode were analyzed. Results from tests of different initial densities show that there is an optimal density index for excess pore-pressure generation. Moreover, observed failure phenomena show that the failure mode was dependent greatly on the grain size and flume angle. In fact, flowslides were initiated in the tests on finer silica sand (no. 8) when the flume angle was 30 degrees, while in other tests, just retrogressive sliding ap-peared.