The development of eco-friendly lubricant additives is of great significance for environmental protection. In this study, cellulose nanocrystal (CNC) was prepared and added into four lithium-based greases with different consistencies and apparent viscosities. The results suggest that the concentration of doped CNC should be limited to a certain value (5 wt% to 10 wt% depending on the grease type) to avoid excessive variation on the basic physical properties of greases. In the pristine grease with low consistency (NLGI grade 1) the incorporated CNC could be effectively transported through the fibrous network and deposited on the sliding surface. Through the surface mending and protection by CNC additive, together with the strong hydrodynamic effect of the high viscous base oil (220 cSt at 40 degrees C), the biggest reduction of friction and wear was achieved by 16% and 29% respectively. This study reveals the lubrication mechanism and application strategy of CNC in greases.