Abstract: The classical methods for estimating from samples the unknown parameters in the distribution function are the moment method and the maximum likelihood method.However, these methods are not always effective when they are used to estimate the three unknown parameters.In this paper, for the sake of estimating the three parameters of the modified Weibull distribution function proposed previously, the existing graphical estimation method has been modified, and the united-estimation method has been suggested in addition.The modified graphical method does not need to use the special probability-paper, nor does it need to trace points over and over again. The united method is a combination of the moment estimation and the maximum likelihood estimation.By estimating the parameters of a vast amount of random samples, it has been shown that the united method is more effective than the standardized variate estimation method proposed by Talreja recently.