Based on detailed field investigation, this paper systematically analyzed the extensional tectonics in the YiwulüMountain area, western Liaoning, and found that the Yiwulüshan metamorphic core complex（mcc）had developed before Waziyu ductile shear zone formed. It has a threelayer structure with an incompletely annular detachment ductile shear zone, which mostly occurred in the basement metamorphic rocks and was truncated by later Waziyu shear zone in the west. Analysis of the mesostructures, microstructures and quartz Caxis fabrics indicates that lower plate, detachment shear zone and the bottom of the upper plate has uniform toptotheSSW shear sense. LAICPMS zircon UPb dating results of the synkinematic leucogranitic dikes from the detachment shear zone and its lower plate show that the Yiwulüshan mcc formed between 157 Ma and 147 Ma, suggesting that the area was under an extensional regime during the Late Jurassic. The revision of the Yiwulüshan mcc indicates that synchronous widespread magmatism in the Yanshan structural belt was under a regionally extensional regime, which implies the northern part of the North China Craton might destructed from Late Jurassic and reached peak period in Cretaceous throughout the whole eastern North China Craton.