Abstract: The XinBarag Right Banner ductile shear zone was firstly found in the western part of northern Da Hinggan Mountains.According to the outcrop, the ductile shear zone should be an overthrust-sinistral strike slip.The latest geological body in the ductile shear zone is Early Triassic granodiorite and its southeast part is covered by Jurassic strata including the undeformed Early Jurassic Chaihe Formation.SHRIMP zircon U-Pb dating shows that the weighted age of the Early Triassic granodiorite is 249.3±4.1 Ma.Integrated with the outcrop geological characteristics of the study area, the authors hold that this large-scale ductile shear zone was formed under an active continental margin setting of the Mongolia-Okhotsk Ocean which subducted southward to the Eergu'Na Block during the Late-Middle Triassic period.A comprehensive analysis of the regional ductile faults shows that the XinBarag Right Banner ductile shear zone is the southward extension of the NE-trending Eergu'Na River-ALongshan ductile shear zone, and this new discovery provides basic data for the study of the regional ductile deformation structure and tectonic evolution in Xing'an-Mongolian Orogenic Belt.