Genetic differentiation among ethnic groups of China is analysed and discussed on the basis of gene frequency data of red cell blood groups. The degree of differentiation, measured by FST analysis of Wright, shows that the MNs system has much higher FST values than those of ABO system. This result is quite different from that calculated by Cavalli-Sforza (1966) on a worldwide basis. Generally speaking, the Rhesus, Lewis and MNSs systems are more useful in anthrological and human genetic studies on Chinese populations for they have a great variation. The genetic difference within and between 12 ethnic groups, namely, Han, Hui, Mongolian, Uygur, Zhuang, Yi, Korean, Manzu, Bai, Tibetan, Dong and Gaoshan, are also analysed. On the basis of gene frequencies of 15 blood group loci which are all polymorphic at least in one of 12 above-mentioned ethnic groups, the net coden difference between populations is calculated by method of Nei and Roychoudhury (1974), and phylogenetic dendrograms are constructed on the basis of net coden differences. Results obtained with the method of gene differentiation coefficient of Nei (1973) and that of Shannon information measure of Lewontin (1972), indicate that the genetic difference between ethnic groups of China accounts for only about 2% of the total genetic variation and is much smaller than that within ethnic groups. Genetic difference between three races of human being is also discussed by taking the Han living in northern China, as typical Mongoloids. On the basis of analysis of gene frequency data of 15 blood group loci, it is concluded that Mongoloids are genetically closer to black people than to white people.