Drop size distribution (DSD) observed at the ground depends on complex microphysical processes that progressively transform ice or water clouds particles aloft into rain below. DSD reflects the physics of rain and is the fundamental property for radar rainfall remote sensing, because it rules the relationship between the radar reflectivity factor Z and rainfall rate R. To better understand the microphysics of the summer precipitation, DSD measurements collected with Parsivel disdrometer are carried out at Beijing Weather Observatory in July to August 2007. The DSD characteristics and the relations among the radar reflectivity factor Z, rainfall rate R and mass-weighted mean diameter Dm are investigated. Specific Z-R relationship is derived for the whole data set.