Abstract: In order to understand the characteristics of magma activities in the southern part of Siberian plate and their influence on the closing time of Hegenshan Ocean during Late Palaeozoic, the authors studied petrography, zircon U-Pb chronology, zircon Hfisotope and geochemistry of the diorite and monzonitic granite to constrain the genesis and tectonic setting of the rock in Bayan Dulan of Dong Ujimqin Banner, Inner Mongolia. The zircon U-Pb dating results show that the diorite was formed at 319.8±0.9 Ma (Late Carboniferous), and the monzonitic granite was formed at 300.8±1.7 Ma (Early Permian). The ε_Hf(t) values and the crust model ages (t_DM^c) of diorite are14.6-17.1 Ma and 335-466 Ma respectively. The Bayan Dulan diorite is enriched in such LILE as Rb and K and depleted in Ba, Sr, Ta、Nb and Ti. The formation of diorite resulted from partial melting of juvenile oceanic crust mixed with mantle wedge during the subduction of the oceanic crust from Middle-Ordovician to Early-Carboniferous. The monzonitic granite is enriched in K and alkali, suggesting the weak peraluminous feature. The monzonitic granite is also enriched in some LILE and LREE, Rb and K with weak negative Zr anomaly and weak positive Hf anomaly, and depleted in Sr, Nb, Ta, P and Ti, suggesting characteristics of post-orogenic granite. The monzonitic granite was produced in an extension environment,and was formed by post-orogenic evolution after closing of Hegenshan Ocean.In addition, the formation of the rock was caused by partial melting of juvenile continental crust resulting from subducted oceanic crust.