The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet, which belongs to the Baila- Yilashan sub- belt of the the middle Bangong- Nujiang ophiolitic belt. The Peng Co ophiolite is mainly composed of mantle peridotites, cumulates, diabase dikes. Peridotites are made up of harzburgites and lherzolites. Mineral chemistry of the Peng Co lherzolites is characterized by Fo contents (88. 85~90. 33) of olivine and Al2O3 content (4. 26%~6. 60%) in orthopyroxenes. Compared to the primitive mantle, the Peng Co lherzolites have relatively higher MgO contents, lower CaO, Al2O3 and TiO2 contents. The total rare- earth element (REE) contents of the lherzolites are 1. 11×10-6~1. 53×10-6, which are lower than those of the primitive mantle. The chondrite- normalized REE patterns of the Peng Co lherzolites display slight loss in LREE. In the primitive mantle- normalized spider diagram, the Peng Co lherzolites exhibit negative Rb and Zr anomalies and intensively positive U, Ta, Sr anomalies. The PGEs contents of Peng Co lherzolites are between 22. 9×10-9~27×10-9. The chondrite- normalized PGEs patterns of the Peng Co lherzolites are consistent with that of the primitive mantle. Mineral and whole- rock geochemistry characteristics of the Peng Co lherzolites show an affinity to abyssal peridotites, indicating that it may have formed in the mid- ocean ridge setting. Through quantitative modeling, we conclude that the Peng Co lherzolites formed after 5%~10% degree of partial melting of the spinel- phase lherzolite mantle source. The crystallization sequence of Peng Co cumulate is olivine- clinopyroxene- plagioclase. The Mg# value of clinopyroxene in cumulate wehrlites ranges from 86. 92 to 89. 93, and the mean value of Fo in olivine is 84. 45, which differ from those of the MOR- type ophiolite cumulates. The mineral composition, sequence of magmatic crystallization and mineral components of Peng Co cumulate are similar to those of the cumulate formed by the SSZ- type ophiolite in the subduction zone. Therefore, we can draw a preliminary conclusion that Peng Co lherzolites were formed in an environment of mid oceanic ridge and were remnants of the spinel lherzolite zone which experienced a partial melting of no more than 10%. In the later period, due to the intra- oceanic subduction, it experienced the rock- melt interaction, and thus formed the SSZ- type cumulates and harzburgite of high Cr value.