Abstract: Nine mid-night heavy rainfall events during August 2003 over the western part of Sichuan basin were simulated by using an Advanced Regional Eta-coordinate Model(AREM). The mechanisms responsible for the diurnal variation of the rainfall are addressed by analyzing the model outputs. The results show that mid-night rainfall over western Sichuan basin usually occurs under a specific large-scale background circulation,both the orography forcing induced convergence/ascending motion and the unstable layer resulted from the thermal forcing of ground surface contribute to the formation and development of the midnight rainfall over Sichuan basin. During the daytime, the low-level temperature and humidity gradually increase with gradually intensified land-air heat flux exchanges, and reach their maximum values after the noon. In the mean time, the low-level warm/moist northeast flow associated with a cyclonic curvature over the western basin gradually strengthens, and the low-level convergence appears, when the warm/moist northeast flow encounters eastwardfacing mountain slopes in the western basin. The low-level convergence transports the warm and moist air upward, which leads to the increasing of both the thickness and intensity of the unstable layer in the lower troposphere. After sunset, the relative humidity in the lower atmosphere gradually increases due to radiation cooling, which makes the air easier to be saturated and the convective available potential energy (CAPE) to accumulate in the lower atmosphere. In the mean time, the topography induced low-level convergence triggers the release of accumulated CAPE, which finally results in the occurrence of midnight convective rainfall.