The Middle-Upper Proterozoic section of Jixian, Northern China well known in the world owing to excellent outcrop, slight or unmetamorphism, simple deformation and aboundant fossils. The section is composed the Changcheng, Jixian and Qingbakou Systems from bottom to top, the total thickness is about 9200m. The uppper part of Changcheng System is Gaoyuzhuang Formation. And the Jixian System contains, in ascending order, the Yangzhuang, Wumishan, Hongshuizhuang and Tieling Formations. JX12 bedded chert in dolomite was collected from the top of the Gaoyuzhuang Formation i. e. the boundary between Gaoyuzhuang and Yangzhuang Formations. JX16 bedded chert in dolomite was collected from the upper part of Yangzhuang Formation, the distance between the sample locality and the top of this formation is 20m. JX26 bedded chert in dolomite was collected from the top of Wumishan Formitaon, i. e. the boundary between Wumishan and Hongshuizhuang Formations.For the 40Ar/39Ar measurements, irradiations were performed in 49-2 reactor in Beijing. Samples were shieled with 0.5mm of Cd during the irradiation, which had a duration of about 65 h. Ar isotopes derived from K, Ca and Cl has been corrected. BSP-1 hornblende, with a K-Ar age of(2060±8)Ma, was used as the flux monitor.All chens analysed contain excess(40Ar)e, although in variable amounts. And the(40Ar)e showed a strong correlation with the(38Ar)Cl relesed, and hence with chlorine. Based on the plot of(40Ar/39Ar)VS.(38Ar)Cl/39Ar, the intercept was obtained, and then the isochron age was calculated. The thin sections and electron microscope showed that all samples contain second fluid inclusions, hence the data from low temperature steps(≤800℃)were deleted when calculating isochron ages.The isochron ages of samples JX12 and JX16 are(1380 ±24)Ma and(1317±22)Ma respectively. The isochron ages of sample JX26 and its duplication determination result(JX26-1)are(1206±27)Ma and(1208±21)Ma respectively.Based on the four ages above, we assign(1380±24)Ma,(1310 ± 20)Ma and(1207 ± 10)Ma as the boundary ages between Gaoyuzhuang and Yangzhuang Forma tions, Yangzhuang and Wumishan Formations, as well as Wumishan and Hongshui.zhuang Formations, respectively.