The determination of the rainfall intensity-duration (I-D) threshold for landslides holds significant importance in the formulation of natural disaster mitigation strategies. Accurate forecasting of this threshold is imperative for the enhancement of early warning systems (EWS) aimed at averting severe repercussions. This study endeavors to investigate the rainfall I-D threshold for landslides, considering varying slope gradients. To achieve this objective, a small-scale sand slope model was constructed within a laboratory setting, incorporating slope gradients of 20°, 25°, and 30°. These slopes were subjected to a rainfall intensity of 132.9 mm/h over a duration of 60 min, corresponding to a 50-year return period in Central Java, Indonesia. Complementary to the laboratory experimentation, semi-coupled numerical analysis employing Geostudio software was performed to assess the Factor of Safety (FOS) of the slopes concerning the dynamics of pore water pressure (PWP) induced by the rainfall event. The findings indicate that the decline in FOS during the rainfall event is notably more substantial in steeper slopes compared to the milder slope. It can be attributed to the higher value of saturated hydraulic conductivity of sand, resulting the same value water infiltration amongst slope angle variations. As a result, steeper slope tend to become less stable than milder slope in associated with higher value of driving force. Consequently, steeper slope gradients exhibit a considerably accelerated attainment of the rainfall I-D threshold for landslides, rather than milder slopes.