In this paper the chemical composition of 141 Mg-Fe micas from granitoids in Eastern China was interpreted in terms of SUBSTITUTION VECTOR The substitution vector V = (αRe1, αRe2, ...αRen, v) is a vectorial form used to represent substitution, here, aRei(.i=1,2,...n) represents the SUBSTITUTIONCOMPONENT of ion R with valence ei in V, vector. LENGTH L= (ΣαR2ei)1/2, COMPONENT SUM (CS = ΣαRei) and ELECTROVALENT SUM ES = Σei·αRei are three parameters of the substitution vector. The length representsthe intensity of the substitution. The component sum represents the change in ions occupancy, and the electrovaient sum — the change in ions valence in structural sheet or bulk of mica due to the substitution The type of a substitution can be distinguished in terms of its CS and ES values.Mg-1Fe+2 (Mg-Fc substitution vector), Fe-3+2(Rv1+3)2□v1 (Fe-muscovite substitution vector) and Fe-1+2Alv1 Si-1Al1v (Fe-Tschermark’s substitution vector) are the three basis vectors of the compositional space of Mg-Fe micas. It was demonstrated by principal components analysis and graphic method that about 80 percent of the compositional changes in the 141 Mg-Fe micas could be interpreted by the basic substitution 8Mg-1Fe+2 + Fe-3+2(Rv1+3)2□v1, and the Tschermark’s substitution was not related to the basic compositional trend. It was also demoestrated that near 90 per cent of the compositional changes were due to octahedral substitutions respricted within the octahedral sheet, and the natural biotite plane composed of Mg-1Fe+2 and Fe-3+2(Rv1+3)2□v1 Was more suitable for subdivision of Mg-Fe micas. These features are the most important crystallochemical relations of Mg-Fe micas occurred in common conditions in K2Mg6Si6Al2O20(OH)4 (phlogopite), K,Fe6Si6Al2O20(OH)4 (annite) and K2Al4Si9Al2 O2U(OH)4 (muscovite) are the most important ideal end-members, which situated in the preceding natural biotite plane. K2Fe4Al2Si4Al4O20(OH)4 (Fe-eastonite), K2Mg4Al2Si4 Al4O20(OH)4 (castonite) and K2Al3Fe+2Si7AlO20(OH)4 (phengite) are the another three ideal members related to Tschermark’s substitution. K2Fe3.75+2 Al1.50 Si6Al2O20(OH)4 (ideal siderophyllite) and K2Fe2.40Al2.40Si6Al2O20(OH)4 (ideal Al-siderophyllite) are not the ideal end members, but only the ideal minerals in the phlogopite-annrenom.