Modification of structure and morphology of polymers has attracted great interest, because their physical properties such as mechanical strength and thermal property strongly depend on their structure and morphology. In particular, the structural change associated with crystallization drastically modifies their physical properties. In this work, we report on the intense-terahertz-wave-induced crystallization of poly(lactic) acid with the terahertz free electron laser (THz-FEL). The moderate THz-FEL irradiation causes the α-phase crystallization in the PLA thin film. This result indicates that the THz-FEL is one of the promising methods to control the morphology and structure of polymeric materials.