Abstract: The Zhangjiakou formation in Western Liaoning, which was discovered and established on the basis of 1:250 000 mapping of Jinzhou and Jianping sheets by regional geologic investigation, is mainly distributed in the east margin of Jinlingsi-Yangshan basin, the west margin of Beipiao basin, Sijiazi basin, Tieyingzi basin and Sanshijiazi basin in Lingyuan along a NE-trending fracture zone. The formation consists mainly of pyroclastic breccia, rhyolite and tuff, unconformably overlying the Tuchengzi formation (or older strata) and underlying Yixian formation. It was formed in the transitional environment of compressional orogeny and extensional structure, as the result of the transition of Paleo-Asian tectonic domain and the Marginal-Pacific mobile belt. The formation belongs chronologically to Late Jurassic epoch.