The role of surface boundary forcing over South Asia in the Indian summer monsoon circulation is analyzed by a series of sensitivity experiments within a regional climate modeling framework. The components of surface boundary forcing that impacted the modeled monsoon circulation are: land use property, land-sea contrast, and orography over South Asia. The land use properties including vegetation and soil conditions do not significantly affect the simulated seasonal rainfall, but they do play an important role in reproducing its intra-seasonal variability. The land-sea contrast is found to play a role in monsoonal precipitation over India and southern China by enhancing the large-scale flow in the lower troposphere north of 10¿N. Orography over South Asia affects the interhemispheric gyre circulation, and its impact penetrates up to the mid-troposphere. This study confirms that the regional climate model is a valuable tool for identifying the mechanisms associated with monsoonal circulation.