This work examined friction-reducing and antiwear behaviors of multialkylated cyclopentanes (MACs) oil with Mo and W nanoparticles as additives under vacuum condition (∼10⁻⁴Pa). The tribological evaluation was conducted on a vacuum four-ball tribometer. The results suggested that the MACs oil exhibited the transient high friction in vacuum, resulted into severe adhesive wear to the steel friction pairs. The Mo and W nano-additives could effectively eliminate the severe adhesive wear. When added with Mo nanoparticles, the lubricating life of MACs oil could be prolonged due to the retarded decomposition of MACs, which should be ascribed to the formation of Mo metallic boundary films on steel ball surfaces that was confirmed by the surface analysis. As for W nanoparticle additives, the rolling effect as nano-bearings and the abrasive wear of larger particles due to the aggregation of W nanoparticles are the dual tribological mechanisms.