This study investigated the effect of 1,3:2,4-dibenzylidene-D-sorbitol (DBS) on the thermal behavior and crystal structure of poly(L-lactic acid) (PLLA) by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Different PLLA crystal structures (alpha- or alpha'-form crystal) were found by the change of the DBS amount and PLLA crystallization temperature. The order and regular alpha-form of PLLA was favored as the DBS component was added. The alpha-form crystal formed more at lower temperatures. Therefore, the disorder-to-order (alpha'-to-alpha) phase-transition temperature of PLLA was found to shift at lower temperatures as more DBS content was added. In the proposed structure of PLLA with DBS, the DBS molecules are stacked together through pi-pi interaction to form a strand with the PLLA molecules by hydrogen bonding. Because of these interactions, PLLA could have a more regular structure with the addition of DBS. In addition, the equilibrium melting point and glass-transition temperature of PLLA were not significantly changed with the addition of DBS. However, the addition of DBS changed the crystal structure of PLLA and thus affected the crystallization rate of PLLA.