Overland runoff is the primary force leading to soil erosion. Estimating runoff is a key step to predict the amount of soil and water loss and evaluate the efficiency of soil and water conservation practices. Runoff curve number method, an empirical model to estimate the storm runoff, was developed by USDA SCS. The curve number method is simple and predictable, especially for ungauged catchments. The purpose of this paper is to present the principle of the method, calculate the CN on the basis of data from 25 different ly covered unit plots on the loess plateau, and to analyze the relationship between CN and its relevant factors.At last, the predictive efficiency of the method is assessed.