Abstract: Study on the petrology, geochemistry and LA-ICP-MS zircon U-Pb dating of the gabbro from Biliutai area in Eastern Inner Mongolia shows that the gabbro was formed during Early Triassic of(249±2.9) Ma, belonging to calk-alkaline series, with SiO₂ of 46.43%-50.77%, Al₂O₃ of 11.27%-16.32% and TiO₂ of 0.20%-0.37%, high-Na (Na₂>K₂O, K₂O/Na₂=0.18-0.56)and high Mg^#(75.08-78.93). The chondrite-normalized REE patterns and primitive mantle-normalized trace element spiderdiagrams show a slight differentiation between LREEs and HREEs featured by(La/Yb)_N(2.32-3.99) and (Ce/Yb)_N (2.45-3.45). The rocks are enriched in large ion lithophile elements(LILEs, such as Rb, Ba, Th, U and K), whereas depleted in high field strength elements (HFSEs, such as Nb, Ta, P, Ti and Zr), with the characteristics of subduction zone setting. Together with the regional geological information, the parental magma of gabbro might be the product of partial melting of lithospheric mantle, which were metasomatized by the fluid from the collisional orogenic belt between Siberian and Sino-Korean palaeoplates.