Comparing the Sm-Nd model ages of the fractionation product to those of source rocks, the authors found that the differences between them are much larger than those calculated through the error equation of Nelson and DePaolo (1985). After rederiving, it is proven that the error e-quation of Nelson and DePaolo (1985) lost a mutiple factor-1/(?). The correct error equa-tion of the Sm-Nd model age caused by fractionation is given: Err tDM Be-cause difference between the model ages of fractionated product and its source rocks is larger than what Nelson and DePaolo expected, the significance of granitoid model ages should be considered deeply, which might not represent the real time of crust -mantle fractionation.