A model of mechanical milling of powders is proposed. It is demonstrated that, during milling, the energy is partially spent in generating microstresses, which hinders the milling process. The model is compared with experiment for tungsten carbide (WC) powder. The average particle size and the value of the microstrains in the as-milled powder are determined from the broadening of x-ray diffraction reflections. Particle size is also evaluated by scanning electron microscopy and sedimentation analysis. It is shown that, with all other things being equal, upon grinding in a planetary-type mill, the resulting particle size is less, the greater the angular velocity of the mill rotation, the longer the milling time, the lighter the weight of the powder milled, and the smaller the initial powder particle size.