The crystallization behavior and crystalline morphologies of poly [(S)-lactide] (P[(S)-LA]) in thin films crystallized isothermally at over 160 degreesC were characterized by transmission electron microscopy and atomic force microscopy (AFM). The dendritic crystal and hexagonal crystal were formed in thin film with thicknesses below 30 nm or over 50 rim, respectively. The crystal structures of dendritic and hexagonal crystals were identical, suggesting that the crystalline morphology of P[(S)-LA] is strongly dependent upon the film thickness. In situ observation of the crystal growth in the P[(S)-LA] thin film at 165 degreesC from the melt was carried out by using temperature-controlled AFM equipped with a heating stage. The initial stage of crystallization and development of lamellae were successfully observed during isothermal crystallization at 165 degreesC. The first forming crystal showed the edge-on orientation, and grew to S-shaped edge-on lamellae. Dendritic flat-on crystals were developed from the S-shaped edge-on lamellae. The growth rates of flat-on and edge-on lamellae were almost identical.