The pseudotachylite developed along the Keketuohai—Ertai fault zone in Xinjiang, northwest China, is one of the friction—meltderived tectonites by the rapid faulting of ancient seismic zone , which is testified by the characteristics of microstructure, e.g. the presence of glassy pseudotachylites and differently morphological microlites, such as skeletal, dendritic, acicular and globular in the matrix of pseudotachylites. The bulk ages of both KAr and 40Ar/39Ar stepheating of the pseudotachylite matrix yielded 282.8±3.8 Ma and 276.4±7.4Ma, respectively; and the ages of UPb of zircons from the two granitoids being the wall rocks of the pseudotachylite are 390.4±1.0 Ma and 396.3±0.7 Ma, respectively. These results show that the pseudotachylites from the Keketuohai—Ertai fault zone formed in the early Permian period , and the occurrences of pseudotachylite may probably indicate the most earliest paleoseismic event during the early Permian times along the Keketuohai—Ertai fault zone. The Fuyun magnitude 8.0 earthquake in 1931 is just one of the multiseismic activities along the Keketuohai—Ertai fault zone, which has no any genetic relationship with the pseudotachylites developed in the fault.