Due to the fact that conventional data is insufficient over the ocean, satellite observations are the best tool for analyzing mesoscale convective systems (MCSs), which frequently grow and develop over the ocean. In this paper, an objective potential index (OPI) is developed from satellite data. Since the OPI includes two kinds of information-the magnitude and the evolution of the air-sea interaction over the ocean, it is suitable for surveying areas where MCSs may develop.Various air-sea parameters were derived from SSM/I microwave and GMS-5 infrared channels covering an area from 15 N to 30 N, and 110 E to 125 E in May and June 1997 to 1999. These parameters were then combined to produce the OPI. The results of this paper show that an OPI value larger (smaller) than 0.5 implies a higher probability that the GMS-5 IR1 brightness temperature will be smaller (larger) than 235 K. It may serve as a good reference in pointing out areas where convective clouds may occur and develop into MCSs.