In this study, we investigate the relationship between the precipitable water vapor (PWV) variability and cumulus convective activity over the Tropical Western Pacific, using the data from the GMS split-window observation. Because of the frequent observation by GMS (every 1 hour), we can conduct a detailed analysis of PWV, which can have a significant impact on the cumulus convection. We modify the split-window method of Chesters et al., which calculate PWV, and compare the derived PWV with the observation by microwave sensor to get a correlation coefficient of 0.6. Using the modified method, our analyses show that deep convection develops as PWV increases, and convective activity is suppressed when PWV is small.