The change in crystal size during high-energy ball-milling of hematite and zinc sulphide powders with initial average crystal size of 8 nm and 4 nm, respectively, has been investigated by X-ray powder diffraction and transmission electron microscopy. It is found that the crystal size increases with ball-milling time. This result shows that the steady-state grain size in ball-milling experiments of bigger particles can be explained by an equilibrium between fracturing and crystal growth. The kinetics of crystal growth during ball-milling of nanoparticles is discussed with reference to a simple empirical model.