Based on the Rb-Sr isotopic study of the granitoids in the Hengduan Mountains, the classification andgeologic significance of whole-rock Rb-Sr isochrons as well as the factors influencing homogenization of theisotopic systems are discussed. Usually. there is no good linear correlation of isochrons for diorites andalkali-rich intrusive rocks (including alkaline rock and alkalic granite). But by means of the external isochron ofmonominerals from the rocks. isochrons with good linear correlation as well as ages and Sr initial ratios with ahigh confidence can be obtained. In order to obtain a satisfactory isochron, the discriminant parameters shouldbe calculated from the Rb/Sr ratios and estimated ages of samples. Only those that meet the requirements canbe used as the Rb-Sr isochrons. The quality of an isochron should be judged from three factors, i.e. correlationcoefficient. MSWD and homogenization degree.