Extreme and/or prolonged rainfall events frequently cause landslides in many parts of the world. In this study, infiltration of rainfall into an unsaturated soil slope and triggering of landslides is studied through laboratory model (flume) tests, with the goal of obtaining the triggering rainfall intensity-duration (I-D) threshold. Flume tests with fine sand at two different relative densities (34 and 48%) and at slope angle of 56.5 degrees are prepared, and rainfall (intensity in the range of 18 to 64 mm/h) is applied via a mist sprinkler system to trigger landslides. Soil water characteristic curve and hydraulic conductivity function of the fine sand are also presented. In flume tests, suction in the soil is measured with tensiometers, the progress of wetting front with time and deformations in the soil are also measured. Some of the findings of this study are: for the fine sand used in this study (a) the failure mechanism is infinite-slope type (mostly translational), and the failure surface is generally coincident with the wetting front or is in its vicinity, (b) the deformations leading to a landslide occurred abruptly, (c) both relatively high-intensity-short-duration rainfalls and relatively low-intensity-long duration rainfalls triggered landslides, (d) the shape of the I-D threshold is demonstrated to be a bilinear relation in log intensity-log duration plot, (e) below a certain rainfall intensity landslides are not triggered, (f) the effect of relative density of the soil on the I-D threshold is demonstrated by physical laboratory tests (as the relative density of the soil increases, the triggering rainfall intensity-duration threshold moves to larger rainfall events). The results of this study could be useful for accurate numerical modeling of rainfall-triggered landslides.