The method of measurement a radar rainfall is studied using both a standard Z-R, B=200 and β=1.6, and a calculated Z-R. A temporal average of radar rainfall shows a good statistics between the radar rainfall (RR) and the gauge rainfall (RG). The calculated Z-R parameters in September 2009 are 18.51 and 1.96 for B and β respectively. Using a calculated conversion factor (C.F) from the calculated Z-R shows a good result of validation of RR and RG. Rainfall pattern shows a high rain rate and high standard deviation of daily mean of rainfall near the Annam range
