A Meiyu heavy rainfall event during 27-28 Jun 2002 is investigated by using GMS-5 satellite infrared images. It shows two stages of mesoscale convective system (MCS) activities with intensive precipitation. In the first stage, a circular-shape MCS moving from the northwest to southeast produces heavy rainfall in Henan Province of the central China; in the second stage, the southerly moving MCS develops and forms a persistent elongated convective system (PECS) which produces torrential rain over the Yangtze River basin. In order to understand why the cloud top shape of the MCS changes from circular to elongated, and what atmospheric processes are related, the NCEP/NCAR 1°×1° reanalysis data are filtered for scale separation. Results show that the mesoscale disturbances output by the Shuman-Shapiro filtering can well correspond to the area where the cloud top temperature is less than or equal to -52℃. Both the first stage circular-shape MCS and the second stage PECS have a similar structure with convergence at lower levels and divergence at upper levels in the troposphere. But their organizations are different: the former is mainly a mesoscale cyclone, while the latter is a mesoscale convergence zone, both at lower levels. This organizing differences cause the MCS cloud top evolution from circular to elongate.