The Laangcuo ophiolite massif situated in southwest Tibet and the western part of the Yalung Zangbo Suture Zone (YZSZ) consists of mantle peridotites and intruded basaltic dikes. The Laangcuo peridotites exhibit a typical porphyroclastic texture as well as melt impregnation and metasomatism textures. Cr# values of spinel in peridotites vary broadly from 0.32to 0.70. Most of samples are LREE_enriched ones associated with increasing HFSE, but a few samples are LREE-depleted. The former underwent a high degree of partial melting (15%～23%), whereas the latter experienced a slight partial melting (～10%). These features show that the Laangcuo peridotites were formed by partial re-melting of MORB-type depleted peridotites during the subduction and reformed by interaction between the melt and the residual mantle, but they were not residues of pyrolite formed directly after the ngle-phase partial melting. As a result of different degrees of mixing and metasomatism between the melt and the residual mantle, peridotites of different degrees of maturity were formed.Petrographic and geochemical studies of the Laangcuo ridotites provide new evidence for the formation and evolution of the Tethys oceanic basin represented by the YZSZ ophiolite zone.