Under the assumption that the raindrop size has a Gamma distribution,random function is employed to model and investigate the effects of the variation in the size distribution and rainfall attenuation on the precipitatlon error measured by a common radar and biline polarization radar. Results show that the effects of the latter are smaller as opposed tothe former,and the rainfall attenuation causes the polarization measurement to be overesti-mated for light rain and to show no big influence for heavy rainfall.