Zircons from the granites in the Mohe region, northeastern Da Hinggan Mountains are euhedral in shape and display oscillatory zoning in CL image, implying their magmatic origin. LA- ICP- MS zircon U- Pb dating results indicate that these rocks were formed in the Early Ordovician (481±5Ma). Lithogeochemcial analysis shows that these rocks belong to the calc- alkaline, slightly peraluminous series granitoids, characterized by high SiO2 contents (71. 42%～72. 57%), high alkali contents (8. 11%～8. 76%) but low MgO contents (0. 52%~0. 54%). The 10000Ga/Al ratios in these rocks vary from 2. 73 to 2. 85, Zr+Nb+Ce+Y contents 374~495×10-6, and the temperature of magma formed is 801~897℃, all these characteristics indicate the rocks belong to aluminous A- type granite. The granites are characterized by the low Sr(150×10-6~205×10-6) and low Yb (0. 89×10-6~1. 07×10-6) and δEu values (0. 24~0. 35). The rocks have Rb/Sr ratios (1. 18~1. 38), Ba/La rations (8. 7～12. 4), Mg# rations （23. 3~30. 3）and low contents Ni （1. 51×10-6~5. 59×10-6）, Cr（6. 6×10-6~13. 4×10-6）, V （20. 0×10-6~21. 6×10-6）. The in situ zircon εHf(t) value varies from -2. 22 to 5. 57 and the twostage model age (TDM2) varies from 1021 to 1453 Ma, suggesting that the primary magma was derived from partial melting of juvenile crustal materials. It can be concluded from regional tectonic evolution and discrimination diagram of tectonic setting that the granites formed in the postorogenic extension setting due to agglomeration of Siberia block and Erguna Massif.