Solid-state polymorphic transitions of organic compound crystals occur during mechano-chemical processing in a planetary ball mill, but the kinds of mechanical energies that influence these transitions are not known. This study examined the solid-state polymorphic transitions from gamma-glycine (Gly) crystals to alpha-Gly ones by collision, friction, and compression energies in order to elucidate the mechanisms involved. When friction and compression energies were applied to gamma-Gly crystals, the solid-state polymorphic transitions to alpha-Gly crystals were observed. The changes in the transition ratios from gamma-Gly to alpha-Gly crystals with time were analyzed by use of Avrami's equation, and the solid-state polymorphic transitions were found to be controlled by the phase boundary in the case of friction energy and by diffusion in the case of compression energy. Collision energy alone did not cause solid-state polymorphic transitions, but the combination of collision and friction energies caused the transitions. From the analysis of the changes in transition ratios with time, the solid-state polymorphic transitions are thought to occur initially by nucleation of alpha-Gly crystals within the gamma-Gly crystals, and then by crystal growth of the alpha-Gly nuclei.