Abstract: There exists controversy concerning the time when the EW-trending strike-slip faults were active in the middle segment of Yanshan. Direct dating of the dolerite dyke that intruded in the fault might give the upper limit of the age. Secondary High Resolution Ion Microprobe U-Pb (SHRIMP U-Pb) and Ar-Ar dating of the dolerite dyke in the Miyun-Xifengkou fault belt yielded zircon SHRIMP U-Pb ages of 290-125Ma and hornblende Ar-Ar plateau ages of 123.5±1.5Ma. The Ar-Ar ages, which are consistent with the youngest U-Pb age, represent the cooling age of the dyke and set an upper limit of the active age of the fault. The scattered U-Pb ages are all inherited zircon ages which correspond to five regional magmatic episodes of the Yanshan orogenic belt.