No.3 pegmatite vein in Koktokay is famous for perfect magmatic differentiation. The authors collected 6 molybdenite samples from the margin zone of No.3 vein whose geological features indicate that there existed hydrothermal metasomatism and that molybdenite was formed simultaneously with or a bit later than the margin zone. The Re-Os dating results of 6 molybdenite samples indicate that the isochron age of (208.8±2.4) Ma(MSWD=0.89) is consistent with the model age of (209.9±1.3) Ma(MSWD=0.69) within error limits, implying that decoupling of Re-Os within molybdenites can be neglected, and that the results obtained by the authors can represent the time of molybdenite deposition from the fluid. Based on studying previous chronologic results, the authors hold that the formation of No.3 vein began at 209 Ma, namely in the late Middle Triassic period.