Abstract: The samples of mylonite were collected from the Guozhacuo fault, situated in the southwestern end of Altyn Tagh fault zone, which extends from Guozhacuo to Longmuco and then to Kongkashankou. The plateau ages of the syntectonic biotite contained in the samples by means of 40Ar/39Ar isotopic dating are listed as follows: 105.9±0.5Ma, 121.5±0.6Ma, 150.4±0.9Ma, which indicated that the ductile deformation of early stage occurred in the Late Jurassic and Early Cretaceous in Guozhacuo fault, and they were consistent with main stages of activities of Altyn Tagh fault zone. Consequently new evidence has been provided for the extension of Altyn Tagh fault zone towards west. That is to say, Guozhacuo fault has the consistent characteristics of deformation geochronology and tectonic attribute with Altyn Tagh fault zone, and the former is the western extension of the latter, both of them are of the same tectonic belt and are also the results of impulsive tectonic activity caused by northward movement of Lhasa block.