The parameters, a . b . c, of Weibull equation were calculated by using data of development of groundnut rust accurring in many years. Stepwise multiple regression analysis showed that the location parameter a and the scale parameter b can be calculated by the number of days for disease to reach at 0.5, 1, 5, 10% and the time intervals of two points among these ones. The shape parameter c can be calculated by the data (time and disease) of the four points and parameters a and b with the weibull equation. According to parameters a, b and c, disease development can be calculated with Weibull model and, if continuing on making investigation of disease development , we can use more data to estimate the parameter c and thus make the forecasting more accurate.