Abstract: The Altun complex,having a Sm-Nd isotopic age of 2789 Ma,is composed mainly of felsic gneiss,basic granulite and amphibolite,which form the old basement of the southern partof the Tarim basin. With respect to the mode of formation and geological age the Altun complex is different from the Tog complex making up the basement of the northern part of thebasin. Such difference in nature of the basement controls the sedimentation,tectonic evolutionand oil-gas accumulation in the southern and northern parts of the Tarim basin.