In this study, a series of large-scale slope failure experiments with torrential rainfall was carried out and simulated using Discontinuous Deformation Analysis (DDA). Since DDA have not been coupled with flow calculation at present, the rainwater infiltration and rising groundwater level inside the slope was simulated separately using saturated-unsaturated flow analysis by Finite Element Method (FEM). The flow simulation results by FEM were incorporated in the DDA mechanical simulation as reduction of the shear strength and increase of the self weight. In the 2-D DDA model, octagonal blocks of 11.8 cm on average were used and rainfall boundary condition (steady rainfall of 100 mm/hour) was applied to the ground surface. The DDA simulation results were compared with the ones obtained from the rainfall experiments and the applicability of DDA to the slope failure/ collapse caused by torrential rainfall was also discussed. The simulation results also show that groundwater plays important roles for the failure process of soil mass, especially rainwater infiltration mechanism was significantly affected by unsaturated soil properties.