Igneous rocks have been divided into three major petrogenetic series: the subalkaline, the alkaline and the strongly alkaline series. A newe method of distinguishing them is proposed in the paper, based on the analysis of pro- perties of major rock-forming oxides. Among oxides comprising igneous sock, SiO2 is an acid oxide, K2O and Na2O are strongly alkaline oxides, Mg0 and Ca0 are mildly alkaline oxides. During fractional crystallization, SiO2 first combines with Mg0 and Ca0 to form forsterite, diopside and enstatite etc. and the seperatian of these mine- rals doesn't change the alkalinity of the magma considerablely. It is discove- red that in the diopside and enstatite SiO2 wt% are approximately equal to (CaO+ 1.5Mg0) wt%.Hence, the relative proportion of (SiO2-Ca0-1.5Mg0) wt% with alkali in rocks can be used to determine the alkalinity of rock series. Boundaries amomg suballLaline series, alkaline series and ;strongly alkaline series are presented in the (SiO2-CaO-Mg0) wt%o υs. (K20 +1.5Na20) wt% diagram based on 263 groups of sock composition data from recent text books. The new method is more accurate in comparision with the SiO2-alkali dia- gram given by Miyashiro and by MacDonald and Katsura, especially for the rocks containing very high or very low silica.