Abstract：Cumulates in ophiolites of the Har Tolgoi area, Damao Qi, Inner Mongolia, are represented by the peridotite+pyroxenite+gabbro association, i.e. the PPG type. Boninite is recognized for the first time from gabbro by petrochemical and geochemical studies, which provides petrological evidence for the tectonic environment of ophiolites in the area. Basalt has the characteristics of MORB and is enriched in components (e.g. Th) of the subduction zone. In the mélange zone there occur Devonian island arc-type intermediate and intermediate-acid volcanic rocks, which conformably overlie siliceous pelite. Based on the above-mentioned analysis, the authors suggest that ophiolites in the area formed in a SSZ-type environment, i.e. at the time of initial subduction. New oceanic crust formed above the subduction (fore-arc) zone and gradually evolved to an island arc.