Abstract: The mafic rock is exposed in northern Anhui Province, and some of them are diamondiferous rocks. In order to determine the type of garnets, the authors systematically sampled the garnets in diamondiferous basite in Lan'gan area. The MgO, SiO 2, FeO, TiO₂, Al₂O₃, Cr₂O₃, CaO, MnO values of 62 single crystal garnets in this area were measured by electron microprobe spectrometer. The results show that the chemical formula of garnet group minerals is A₃²⁺B₂³⁺(SiO₄)₃. Group A cations are composed of Mg²⁺, Fe²⁺ and Ca²⁺. Group B cations are composed of Al³⁺, Fe³⁺, Mn³⁺ and Cr³⁺. The trivalent cation is mainly Al³⁺ while divalent cation is mainly Ca²⁺. It is inferred that grossularite-andradite-almandine group is the primary class in the Lan'gan area. The high calcium silicate iron garnet (majorite) was found in 62 samples. According to the composition of these majorities in Lan'gan area, the garnet of the diamondiferous basite has formation pressure of 12.1~12.8GPa, and the depth is up to 300km.