This paper aims to investigate the influence of grain breakage on the position of the critical state line (CSL). Existing measures of the amount of grain breakage are discussed and a new measure named "relative uniformity" based on the evolution of the uniformity coefficient is proposed. Triaxial tests with different stress paths on granular materials made of limestone grains are carried out. Based on the obtained results, a method for determining the modified relative breakage index and the relative uniformity is proposed using plastic work. A relation between grain breakage measures and the position of the critical state line is then proposed for describing the influence of grain breakage on mechanical properties.