Techniques of estimating areal rainfall are firstly reviewed, and the principles of Kalman filter calibration and optimum interpolation calibration are introduced. Based on analysis of the two methods characteristics, the authors present a new method to estimate areal rainfall, i.e., two-step calibration method. At the end, the radar data observed by the Doppler radar (CINRAD/SA) in Hefei and the raingauge data are used to evaluate the precision of the three methods. The results show that not only in precision but also in stability, the two-step calibration method is better than the other two methods.