Erosive rainfall standard,which can pick out erosive rainfall from all rainfall events,has to be determined based on the rainfall samples. How the sample size influences the erosive rainfall standard has not been reported yet. According to the rainfall and runoff data during 1962-1969 attained from Zizhou experiment station in the Yellow river basin,the effects of sample size on the valuesof erosive rainfall standards were studied. To get the stable rainfall standard,large sampleswere needed. But considering the data available and not too much work,5- year data were enough for determining the erosive rainfall standard.