This paper presents whole-rock K-Ar dating results and geochemical data of the Late Mesozoic basalts in the Liaoyuan area, Jilin Province, and geochemical data of minerals from the entrained mantle xenoliths, with the aim of constraining the origin of the basalts and the nature of the Late Mesozoic lithosphere mantle. Combined with the studies on the regional geology, the authors conclude that the basalts formed under the back-arc extensional setting related to the subduction of the Paleo-Pacific plate.