Abstract: TRMM (Tropical Rainfall Measurement Mission) was launched in December 1997, which carried a space radar for the first time and the related data have been made available to the public users since June 1998.The microwave characteristics of rainfall over South China and the Yang tze river basin are investigated by using TMI (TRMM Microwave Imager) data and TRMM-derived precipitation products through Is (Scattering Index) algorithm.The expression of the I_sis regressed by the TMI data on sunny days.For 14 rainfall cases, the relationship between the I_sprecipitation threshold obtained through the "area equal" method and the precipitation area and the coldest top temperature of 85.5 GHz vertical channel is investigated.At the same time, the correlation between I_sand surface instantaneous rain intensity is studied.The result shows that the I_sthreshold has the increasing tendency when the precipitation area increases or the top temperature decreases, and I_s≥60 K is a good index of rainstorm.The instantaneous rain intensity is retrieved, and it is found that I_scan be used to retrieve rain intensity better because of the improvement of data resolution and the temporal and spatial consistency of the data.