The MOR-type Bangong Co ophiolite in northwest Tibet is a tectonic mélange mainly consisting of brecciated peridotites and MORB-affinity basalts. The mantle peridotites are composed of clinopyroxene-bearing harzburgites (cpx-harzburgites) and minor clinopyroxene-free harzburgites classified in terms of petrography, mineralogy and geochemistry. Geochemical data suggest that the cpx-harzburgites were formed via a refertilization process, which occurred at low melt/peridotite ratios (1∶9 to 1∶4) and under the interaction between basaltic melts and clinopyroxene-free harzburgites. The authors performed a Re-Os isotopic study of 9 cpx-harzburgites, which display a relatively narrow range in Os concentrations and Os isotopes, with Os ranging from 2.91×10^-9 to 5.40×10^-9 and ¹⁸⁷Os/¹⁸⁸Os ranging from 0.121 13±0.000 44 (2σ) to 0.128 53±0.000 36 (2σ). Measured Re concentrations range from 0.19×10^-9 to 1.49×10^-9 and ¹⁸⁷Re/¹⁸⁸Os range from 0.169±0.009 (2σ) to 1.833±0.183 (2σ). The positive correlation defined by cpx-harzburgites (five solid circle points) in the isochron diagram (¹⁸⁷Os/¹⁸⁸Os versus ¹⁸⁷Re/¹⁸⁸Os) yields an apparent age of 254±28 Ma. The peridotite-melt interaction at the low melt/rock ratios could result in an obvious increase in peridotite Re abundances and Re/Os ratio, and then the decay of ¹⁸⁷Re to ¹⁸⁷Os produced a modification of the ¹⁸⁷Os/¹⁸⁸Os ratios. Serpentinization seemed to have had little effect on the Re-Os system, which is considered to have been a closed system during the low temperature process. Therefore, this pseudo-isochron age may reflect the melt/rock reaction between cpx-free harzburgites and basaltic melts. In addition, this pseudo-isochron age is consistent with the opening age of Neo-Tethys defined earlier, and hence provides evidence for the initiation of the Bangong-Nujiang Tethys Ocean in Late Perimian-Early Triassic period.