The Yanshan orogenic belt is an important Mesozoic tectonic feature in the north of the North China Craton (NCC). It was formed by the “Yanshanian Orogeny” (Wong, 1927, 1929, Bulletin of Geological Society Of China). The \"Yanshanian orogeny” has two phases (A and B) reflected by two regional angular unconformities. Both phases are characterized by crustal shortening. Phase A is marked by the unconformity beneath the andesite volcanic rocks of the Tiaojishan formation and Phase B by the unconformity beneath volcanic rocks of the Zhangjiakou formation. The end of Phase B represents the transformation of the east part of the NCC from crustal shortening to crustal extension. Although the time gap represented by the angular unconformity marking Yanshanian Phase B has constrained isotopically to be between 145Ma and 126Ma, the precise time duration for Phase B remains to be further refined. Within the central segment of the Yanshan orogenic belt, the Sihetang ductile shear zone is identified as having developed during Phase B of the Yanshanian Orogeny. We have dated the deformation of this shear zone and our work has bearings on the timing of Phase B of the Yanshanian orogeny. The Sihetang shear zone is located to the north of the Early Cretaceous Yunmengshan pluton. Shear sense indicators suggest top-to-the-SSW thrusting. The metamorphic basement, Meso- and Neo-proterozoic cover layers, and Late Jurassic to Early Cretaceous plutons are involved in the ductile shear zone deformation. Many syntectonic dykes are present in the Shihetang shear zone, and their order of emplacement during shearing is established by structural analysis. Dykes are divided into early, intermediate, and late stages, based on their geometries within the shear zone. Zircons from dykes of each stage are selected for dating. LA-ICP-MS zircon U-Pb ages for the three stages are respectively 143.0±2.1 Ma, 140.8±1.4 Ma, and 137.5±2.4 Ma, consistent with dyke emplacement order constrained from structural analysis. We suggest that 143-138 Ma should be the time period for the Sihetang ductile shear zone deformation. Considering the ages of deformed and un-deformed plutons associated with the shear zone, we suggest that the shear zone deformation represent the entire duration of Phase B. Therefore, the time period of Phase B of the Yanshanian Orogeny is 143-138 Ma.