The fragmentation mechanism taking place during dry grinding of poly(vinyl acetate) particles in a shaker bead mill has been analysed using various shape factors permitting a quantitative description of the particles morphology. The knowledge of the size evolution was not sufficient to understand the phenomena occurring in the mill. Several experiments were carried out with different operating conditions and it was shown that the predominant phenomenon governing the fragmentation depends mainly on the intensity of the stress applied to the particles. When this is high, the entire particles are broken while only small fragments are removed when the stress is low. This study permits the definition of the sub-populations of particles created after fragmentation, which can be used for modelling the grinding kinetics.