Abstract: This paper reports LA-ICP-MS zircon U-Pb ages and geochemistry of the granite in Mashan, east of the southern Beishan, Inner Mongolia, with the purpose of constraining its formation age and petrogenesis. The results show that the granitic body mainly consists of granodiorite which belongs to middle-K calc-alkaline series with metaluminous-peraluminous characteristics and high content of SiO₂(64.85%~79.17%), Na₂O+K₂O (5.13%~6.62%) and Na₂O/K₂O>1. In addition, the granite invariably exhibits relatively gentle light rare earth elements (LREE) enrichment with flat heavy rare earth element (HREE) and weak negative Eu anomalies (δEu=0.65~0.91) in the chondrite-normalized REE patterns, depletion of Ba, Nb, Ta, P, Ti and enrichment of Rb, Th, U, K in the spidergram. The LA-ICP-MS zircon U-Pb age of the granite is 281.8±3.2Ma. Based on regional geology, the above characteristics indicate that the Mashan granite was the mixing product of crustal and mantle derived magmas and was formed under the tectonic setting of post-collisional extension in Early Permian. This implies that the east of southern Beishan Mountain turned to the stage of rift in post-collisional extensional period during Early Permian.