Abstract: By numerical simulation and sampling to dropsize spectra of precipitation consisting of polydisperse small spheroid particles, the relationship between attenuation coefficient and radar reflectivity factor at 3 common-used wave lengths has been studied for the incident microwaves which are transmitted horizontally and polarized either horizontally or vertically. The resulted empirical formula is presented and explanations are made in physical sense. The formula can be applied directly into correction for attenuation to radar echoes during data proces sing for quantitative measurement of precipitation.