By controlling the heat-treatment process, three types of lithium greases with different thickener fiber morphologies were synthesized via the saponification reaction. With increasing the cooling rate, the dimension of the thickener fiber decreased. The relationship between microstructure and tribological performance of the lubricating greases was investigated via a rheological method. The results indicated that the fiber dimension determined the level of physical entanglement and the evolution of fiber network in the friction process, further influencing the final lubricity. The grease with the large fiber dimension displayed good tribological performance under low frequency and high load conditions due to its large-scale strongly crosslinked structure. In addition, the grease with the small fiber dimension yielded a low level of fiber physical entanglements and displayed low structure strength and fast response in the rheological test. It could obviously improve the tribological performance under high frequency conditions. Through tuning the microstructure of the thickener fiber according to the lubricating conditions, the tribological performance could be obviously improved.