Biotite is an important rock-forming mineral in most of granites, and its mineral chemistry can reveal the crystallization conditions, petrogenesis and tectonic setting in the magma system. In this study, the authors utilized electron microprobe analysis (EMPA) to determine the mineral composition of biotite grains hosted in the Pailou granite. The EMPA data show that the biotite is poor in Al^Ⅵ (0.128 to 0.395) atomic number and moderate in Ti (0.236 to 0.267) atomic number, and shows siderophyllite affinity. In addition, the biotite is rich in iron and poor in magnesium, with Fe²⁺/(Fe²⁺+Mg) values from 0.985 to 0.989. Petrography and mineral chemistry indicate that the biotite grains are of magmatic origin. Based on calculation of the EMPA data, the crystallization temperature and pressure of biotite are 640～710℃ and 0.9～1.7 GPa, respectively. Besides, it was formed under the condition of low oxygen fugacity, with the log f_O2 values from -18.1 to -17.4. The low oxygen fugacity of magmas can explain the lack of Cu, Au and Mo mineralization, whereas the lack of Sn mineralization may be attributed to the low temperature of magmas. Combined with the study of mineral chemistry of biotite and geochemical features of hosted rocks, the authors hold that the Pailou monzogranite is of A-type and was formed in the nonorogenic tectonic setting.