Abstract: The Targen monzogranite was sheared by the Tuoli fault in the middle of west Junggar and the maximum initial time of left-lateral slip of the Tuoli fault will be limited by the emplacement time of the monzogranite. The sample, light fleshred monzogranite, was collected at N45°45.911′, E83°32.584′, located at the south of the Tuoli City, 50 meter southeast to the fault plane. The U-Th-Pb isotope data of 29 grains, i.e. 31 spots, of zircons in the sample were obtained by using the SHRIMP Ⅱ in Beijing SHRIMP Center. In all of the data，the 206Pb/238U ages of the zircons are between (279.5±2.7)Ma~(337.3±2.7)Ma, with the Th/U ratios of 0.31~0.88, U contents of 65×10-6~384×10-6 and Th contents of 23×10-6~142×10-6. According to the observation of the very similar CL images of the zircons, the weighted average value of 206Pb/238U age of (295.8±2.5) Ma was obtained by the calculation using 13 among the 31 data, which is interpreted as the emplacement time of the Targen monzogranite. Based on the fact of left-lateral slip sheared and the emplacement time of the Targen monzogranite，it is clearly indicated that the initiation of left-lateral slip of the Tuoli fault took place after 295 Ma.