Critical rotation speed of dry ball-mill was studied by experiments and by numerical simulation using Discrete Element Method (DEM). The results carried out by both methods showed good agreement. It has been commonly accepted that the critical rotation speed is a function of a ball radius and a jar diameter. The results of the present work, however, show that the critical rotation speed significantly depends on ball-containing fraction in jars, and approaches a value asymptotically as the ball-containing fraction approaches to one. This asymptotic value is equal to the traditional critical rotation speed. The effect of individual rotation of the ball of the jar is significantly small to centrifugal motion.