The goal of this paper is to evaluate the feasibility of the Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI) technique for the mesoscale convective systems (MCSs) prevailing over the northern part of the South China Sea in the Mei-Yu season. The rain rate retrievals using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager were used as the oceanic validation data. The global rain threshold, 235 K, and the global GPI coefficient, 3 mm/h, of the GPI technique were adjusted using a method combining the microwave and infrared rain observations from the TRMM satellite. Once the TRMM rainfall observations were not available, IR rain observations from the Japanese Geostationary Meteorological Satellite 5 (GMS-5) provided rainfall information off shore using the adjusted GPI formula. During a period with uumerous rainfall (from 1 May to 12 June 1998), a total of 60TRMM overpasses were computed in the statistics. Nineteen of these overpasses contained active convections. The average IR rain threshold is 216 K for cases with a spatial averaging scale of 1°. This technique cannot provide adequate rainfall information under such spatial and temporal requirements for the overpasses without active convections. The optimal advantage of the adjusted GPT technique is the simplicity of its calculation and that it demonstrates adequate ability for monitoring MCS induced rainfalls.