This paper describes an experimental study on quantitative evaluation of correlation between the structure formed by thickeners and yield stress of greases. In order to evaluate the thickener structures, untreated surface of greases was observed by using an atomic force microscope, and an index was obtained from AFM images to describe the degree of dispersion of the thickeners. The index physically represented both factors of shape and distribution of thickeners. The results for twelve types of lithium soap greases with different base oils and thickener concentrations showed good correlation between the index of the degree of dispersion and the yield stress of the greases. This relationship indicated that uniform distribution of thickeners in greases caused higher yield stress.