In this article, the shear degradation of lithium 12-hydroxy stearate grease will be measured using an in-house-developed Couette aging machine. In this device the shear rate is well defined. The aging is related to the generated entropy density as described in Rezasoltani and Khonsari's work (Tribology Letters, Vol. 56, No. 2, pp. 197–204, 2014). The rheological properties of the aged samples were evaluated using a parallel-plate rheometer. The results showed that there are two aging phases with different degradation rates: a progressive degradation phase at the early stage, followed by a rather slow deterioration afterwards. Based on this observation, an aging equation was formulated to describe the aging behavior of lithium-thickened grease. Atomic force microscopy results of the fresh and aged greases showed that the variation in thickener microstructure provides a good explanation for the lithium grease degradation mechanism: under shear, the original fibrous network is progressively destroyed and becomes fragmented, leading to the loss of consistency and a change in the rheological properties.