Late Jurassic trachyandesites from Tiaojishan Formation in Western Hills of Beijing are similar to adakitic rocks in geochemical characteristics. They contain high SiO2(>56%), Al2O3(16.33%~17.9%),Na2O(3.11%~4.8%), Na2O+K2O(6.11%~8.10%), K2O(1.64%~3.59%) and CaO(3.68%~5.82%), low MgO(<3.0%, Mg#≤0.50), high Sr (554×10-6~1 258×10-6), low Yb(<1.70×10-6)and low Y(<21×10-6), and have high Sr/Y, Y/Yb and Na2O/K2O ratios and low Rb/Sr ratios. In addition,they are characterized by strongly fractionated REE and absence of Eu anomaly. It is thought that the trachyan-desites are derived from partial melting of lower-crust calc-alkaline basaltic rock and are related to the underplatingof basic magma. Based on the geochemical features, the depth of magma is less than 50 km.