The phenomenon of solvent-induced crystallization observed in some polymer-solvent pair was well documented. It is generally believed that solvent-induced crystallization occurs during sorption of the solvent. However, this is actually a hypothesis and needs to be proven. To clarify the true situation, the present article investigates crystalline structural variation of polyetheretherketone before and after treatment of methylene chloride. The results indicate that crystallization of the polymer was induced by the solvent mainly when the solvent was desorbed. The possible mechanism is discussed in the light of molecular motion.