The aim of this work is to investigate the effect of molybdenum disulfide on tribological properties of Ultra-high-molecular-weight polyethylene (UHMWPE). UHMWPE/ MoS 2 nano-composites were prepared using in-situ polymerization and Ziegler-Natta catalytic system. Studies showed that, in order to obtain the optimum tribological properties, interlayer distance between nanosheets should be as high as possible.Therefore, the nanosheets were subjected to oxidation using the required oxidants followed by thermal shock and ultrasound. Fourier-transform infrared spectroscopy (FTIR) analysis was used to determine the formation of functional groups which indicate the formation of S═O bond in the molybdenum disulfide oxide. Then, in order to exfoliate and increase the interlayer distance of nanosheets, thermal shock and ultrasonic were conducted Field Emission Scanning Electron Microscopy (FESEM) analyses. Finally, the obtained sample was used as the second support of catalyst in in-situ polymerization process. Then, under specified and constant operational conditions, the synthesis of samples containing 0.5%, 1%, and 2% nanoparticles were conducted. The results of the morphological study with FESEM indicate that the filler is uniformly distributed in the matrix, and X-ray Powder Diffraction (XRD) analysis showed that completely exfoliated nanocomposites were resulted and in low nanoparticle content. Addition of MoS 2 as reinforcing agent had a significant effect on friction coefficient of the nanocomposites. With the addition of 0.5 wt% of MoS 2 , the friction coefficient declined by approximately 35%. Furthermore, negligible decrease in sample's weight after the wear test is an indication of a strong interaction between MoS 2 and the matrix. Scratch resistance test revealed that with the addition of filler, the resistance increased from 23 to 60 MPa.