Surface functionalization can enhance the dispersibility and lubrication performance of molybdenum disulfide (MoS2) nanosheets in lubricating oil. Herein, the MoS2 nanosheets were functionalized with quaternary phosphonium ionic liquid ([P888A]DEHP) via ultraviolet-induced thiol-ene click chemistry. The functionalized MoS2 nanosheets (MoS2-PEMTP/[P888A]DEHP) exhibited dispersibility in polyethylene glycol 200 (PEG-200). Tribological test demonstrated that the addition of 0.5 wt% MoS2-PEMTP/[P888A]DEHP reduced the friction coefficient by 18.85 % and wear volume by 65.59 %. This improved performance originated from both roles of [P888A]DEHP in facilitating interlayers slip within MoS2 nanosheets and forming tribochemical films on contact surfaces. This work exemplifies how to improve the tribological properties of MoS2 nanosheets through surface functionalization.