The high-energy ball milling behavior of a commercially available ZrB2 powder was studied by X-ray diffractometry and transmission electron microscopy. It is shown that ball milling is very effective in obtaining nanocrystalline ZrB2 powder. Further, the crystallite size refinement occurs through repeated brittle fracture, with the final crystal size being limited by a combination of the fracture strength of particles and the compressive stress induced by collision of balls. With this mechanism, the crystallite size first decreases rapidly, then gradually, and finally approaches to an asymptotic limit as the ball-milling time increases. Additionally, the higher the ball-to-powder weight ratio, the faster the decrease in the crystallite size and the smaller the ultimate crystallite size.