Cenozoic corundum-related basalts have been discovered in the western, eastern, central, and northern parts of Thailand. These basalts usually contain various types of inclusion were collected from Bo Phloi, Chanthaburi-Trat, Wichian Buri, Chiang Khong, Nam Cho, and Den Chai basaltic areas. Petrographically, the xenoliths are characterised particularly by ultramafic and feldspathic rocks of wide-ranging modal compositions. Ultramafic nodules, presumed to be form the upper mantle, are usually represented by peridotites, such as spinel lherzolite, spinel-plagioclase lherzolite and spinel harzburgite, although rocks of the pyroxenite series are occasionally represented by olivine websterite and norite. The feldspathic xenoliths, which probably originated in the lower crust, are usually composed of abundant alkali-feldspar with minor amounts of plagioclase, quartz and traces of apatite, amphibole, zircon and opaque minerals. In addition, other types of xenolith, such as gabbro, dolerite, tuff, etc., are locally found in these basalts. Xenocrysts in these basalts comprise mainly olivine, clinopyroxene, orthopyroxene, spinel, alkali-feldspar and minor localised garnet, plagioclase, nepheline, amphibole, apatite and magnetite. These xenocryst types are related predominantly to minerals in the associated xenoliths; however, mineral chemistry indicates that these garnets are pyrope-rich and chemically resemble garnets in periotite nodules from kimberlites. Regarding preliminary thermobarometry, estimated pressures and temperatures of equilibration of spinel lherzolite, spinel-plagioclase lherzolite, spinel harzburgite and olivine websterite range approximately from 11 to 22 kbar and 1170 to 1450^oC, respectively. The pressures indicate that the xenoliths were sampled from depths of ca. 40 to 80 km. The estimated condition probable reflect pressures and temperatures within the upper mantle beneath Thailand in late Cenozoic time (ca. 10-1 Ma ago). A Cenozoic geothermal gradient of ca. 27 ± 12^oC/km is implied by the data.