Based on consideration of both thermodynamic and kinetic features of the subtropical summer monsoon in East Asia, a new index is defined by the moist potential vorticity (MPV) for this monsoon.Variation features of the subtropical summer monsoon over 60 years are analyzed using National Centers for Environmental Prediction/National Center for Atmospheric Research (USA) data from 1948 to 2007.Results show that the new index can well reflect the seasonal, interannual, and interdecadal variations of the East Asian subtropical summer monsoon. Correlation analysis of the new index and precipitation data from 160 stations in China shows that in high-index years, the summer monsoon is strong, and more rain falls in eastern North China, southwestern China, and along the coast of South China and less rain falls in the Yangtze-Huaihe R. basin. In low-index years, the opposite occurs. Lastly, the new index is compared with four established monsoon indices. The new index is found to have an advantage in representing summer rainfall in the Yangtze-Huaihe R. basin.