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This study investigates the roles of tropical Indian-Pacific oceans SSTs in the interdecadal variability of East Asian Summer Monsoon (EASM) during the second half of the twentieth century based on simulations from an atmospheric general circulation model, NCAR CAM3 (Community Atmospheric Model). These simulations are conducted by the observed 1950-2000 monthly SSTs in the tropical Indian-Pacific oceans, tropical Indian Ocean and tropical Pacific, independently. Results show that CAM3 is able to simulate the observed interdecadal weakening of EASM occurred in the mid-late 1970s when SSTs in tropical Indian-Pacific Oceans and tropical Pacific SSTs are determined, but to simulate the interdecadal strengthening of EASM when SSTs in the tropical Indian Ocean are determined. These results indicate that SST anomaly in the tropical Pacific plays a central role in the interdecadal weakening of EASM occurred in the mid-late of 1970s, while SST anomaly in the tropical Indian Ocean plays a converse role in it. As a background, the interdecadal variation of tropical Pacific SSTs has an important effect upon the interdecadal weakening of EASM.