Study on the damage of rice sheath blight occured in different conditions, variety, Napplication, yield level, pathogenous time, and disease level, had been carried out continuously on early season rice in the past six years. A model of multi-factors-yield-loss assessment was set up. The model indicats themost close relationship between the disease index at rice yellow ripeness stage and the rate of yield decrease. According to the model, six experimental varieties were classified into two types depending upon the yield loss level. Field tests in 2 years show that the suggested model is well reliable. Napplieation and yield level showed no-significant effect on the rate of rice decrease caused by the same disease level.