Objectives：This paper provided petrography, LA- ICP- MS zircon U- Pb age and geochemistry of the monzogranite in Ajinguole area of Zalute Banner, Inner Mongolia, and discussed the formation age, genesis and geological significance of the rocks. Methods：Two fresh samples were collected for analyzing the whole- rock main and trace elements in Ajinguole area of Zalute Banner. They were also selected to carry out the zircon U- Pb dating by LA- ICP- MS method. Zircon U- Pb isotope ages of selected samples were performed in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing). During the period, data were processed and plotted by ICPMSDataCal program developed by Liu Yongsheng (2008) and Isoplot program developed by Kenneth R. Ludwig (1999). Common lead was corrected according to Anderson (2002), and the contents of Pb、U and Th in zircon samples were calculated using NIST612 as external standard. The determination of major elements, trace elements and rare earth elements in chemical analysis samples was completed in Beijing Research Institute of Uranium Geology. The melting method of XRF fluorescence spectrometer was used for precise analysis of the major elements in rocks. The content of trace rare earth elements in the whole rock was determined by ICP- MS. Results：Based on the study of occurrence characteristics, microscopic characteristics and geochemical characteristics, the emplacement age was determined and its genesis and tectonic background were discussed. The results show that the U- Pb weighted ages of zircons in the monzogranite are 124±1 Ma and 126±1 Ma, respectively. The content of SiO2 in the monzogranite is 69. 45% to 73. 64%, Na2O and K2O are 3. 16% to 4. 62%, 3. 78% to 4. 37%; Al saturation index A/CNK is between 1. 00 to 1. 25. At the same time, the trace elements such as K、Ba、Rb and other large ion lithophile elements, incompatible elements such as Zr、Th、U、Hf, show enrichment state, on the contrary, high field strength elements such as P、Ti、Nb、Ta, are seriously depleted. Conclusions：The intrusive age of the rocks was the Early Cretaceous. It belongs to I- type granite with high potassium calcium, alkaline peraluminous with high differentiation, which has the genetic characteristics of subduction zone. Considering the background of regional tectonic evolution, we consider that the extension of Zalute Banner in the southern segment of the Da Hinggan Mts. was mainly related to the extensional collapse of the Mongolian—Okhotsk orogenic belt.