Baerzhe granite in Inner Mongolia is a rare rock depleted in 18O. This paper gives the oxygen isotopic values of quartz, feldspar, riebeckite, zircon and whole rock of Baerzhe alkali granite, which seem to be the lowest among these isotopic values in all the alkali granites of China. In the light of isotopic data, this paper deals with geological and geochemical characteristics, the origins of Baerzhe rock body and the 18O-low alkali granites in eastern China. The authors consider that two arguments are relatively reasonable for illustrating the petrogenesis: one is isotope exchange between the rock and the meteoric water, and the other is magmatic degassing of the rock in an open system. The problem of petrogenesis deserves further researches.