Abstract: The rain rate R and radar reflectivity factor Z were calculated based on the raindrop size distribution data of 45 precipitation events measured in Beijing during 2004 by a Disdrometer, which has 20 channels and measures the raindrop diameter from 0.35 mm to 5.0 mm. For different precipitating clouds, the relationships between R and Z, or coefficient a and exponent b, are quite different, that maybe attribute to different raindrop size distributions. At the same time, the Z-R relationship over land is also different from that over the sea. There is a positive correlation between a and b in these 45 events of precipitation. The relationship between coefficient a (or exponent b) and some physical quantities, such as rain rate R, radar reflectivity factor Z, characteristic diameter Dm and raindrop number density NT is analyzed, respectively. When R < 5mm/h, or NT < 1000 m, or Z＜2000 mm6/m3, coefficient a (or exponent b) changes greatly, and when R, or NT , or Z gradually increases, coefficient a (or the exponent b) approaches to a constant value. Both coefficient a and exponent b decreased from April to December in 2004. Moreover, the characteristics of Dm derived from the raindrop size distribution data of 12 precipitations are analyzed, and the results show that there is a linear correlation between R and Z, in other words, exponent b is 1, when Dm is a constant.