The carbon isotopic composition of CO2, CO and CH4, extracted by pyrolysis from spinel lherzolite and black pyroxenolite xenoliths in alkali basalt from Damaping has been measured with GC-MS. The results show that, at heating temperatures from 400 to 1 140°C, the δ13C of CO2 and CO is −22‰–−27‰, and that of CH4 −30‰–−50‰ The data imply that the magma which was generated in the upper mantle by partial melting in the area has experienced the multistage degassing process, and the δ13C values represent the carbon isotopic composition of CO2, CO and CH4 remaining in it, respectively.