Abstract: By investigating the geochronology and petrogeochemistry of monzonitic granite in Ganjin Mountain, Ejinaqi, Inner Mongolia, the intruding age and tectonic background are discussed. The zircon U-Pb weighted average age is considered to be (276.63±0.84) Ma by using LA-ICP-MS method, constraining its formation age to be Early Permian. Lithological and geochemical characteristics indicate that the monzonitic granite belongs to subaluminous-peraluminous and calc-alkaline series, with A/CNK ratios of 0.97-1.15 and σ values of 1.64-1.96. The monzonitic granite is rich in large ion lithophyte elements (LILE, e.g., Rb, Ba, Sr), high field strength elements (HFSE, e.g., Th, U, Zr), and LREE with negative euanomolies on the chondrite-normalized diagram, and its LREEs and HREEs are not obviously differentiated. They share affinities with syn-orogenic I-type granites, indicating they formed in a continental arc setting. It is suggested that the monzonitic granite was generated during late Hercynian movement in Ganjin area. The time that terranes on both sides of the Hongshishan-Lujing rifting belt converged was limited to Early Permian.