The Fanshan ultramafic syenitic complex is located on the northern margin of North China Craton (NCC), and crystallized from a SiO2 undersaturated ultrapotassic alkaline peralkaline parent magma which was originated from an enriched lithospheric mantle source that had previously been metasomatized by melts from carbonated oceanic crust above a subduction zone. The Fanshan complex was emplaced in late Trassic, and is a typical compelx among the alkaline/ultramafic magmatic belt along the northern margin of NCC. Major and trace elements compositions of biotites from the Fanshan complex are obtained using electron microprobe (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA ICPMS), respectively. The biotites from the Fanshan complex have Fe/(Fe+Mg) in the range 0.32~0.57, and total Al from 1.179 to 1.375, and belong to typical biotite. Biotite compositions indicate that the crystallization temperatures of the garnet clinopyroxene syenites are between 680℃ and 780℃, and their oxygen fugacity (fO2)＝10-12.5~10-15. The biotites from the Fanshan complex have relatively low contents of rare earth elements (REE) with total REE contents lower than 0.100×10-6~1.077×10-6, indicating that biotite is not a major carrier of REE of the whole rock. However, trace elements compositions of biotites indicate that biotite is a major carrier of Rb, Ba, Nb, Ta, V, Cr, Co and Ni; whereas the contents of Sr, Zr, Hf, Y, Sc, Th and U of biotites are much lower than those of whole rock. Biotite compositions suggest that the Fanshan complex is characterized by high Fe contents and high oxygen fugacity, which may be the reason why the complex hosts an apatite magnetite deposit.