By using the routine sounding and surface observation data, the circulation features and physical causes for the heavy rain event occurred in the East Side of Qinghai-Tibet Plateau on 8-9 June, 2002 are diagnosed. Furthermore, based on MM4 model output,a mesoscale analysis is made according to moist potential vorticity. The results show that the spatial and temporal variations of moist potential vorticity can give good indication of the development of mesoscale cyclone and heavy rain. The heavy rain is located between the positive and negative moist potential vorticity zones in the lower troposphere, the rapid development of mesoscale cyclone is closely related with the spread downward of disorder moist potential vorticity in the higher troposphere.