The effect of molecular weight (MW) on the polymorphous crystallization and melting behavior of poly(L-lactide) (PLLA) were systemically studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and time-resolved Fourier transform infrared (FTIR) spectroscopy. It was found that the polymorphism of PLLA is not influenced much by MW, and the alpha'- and a-form crystals are produced at low and high crystallization temperature (T-c), respectively, regardless of the MW. However, MW significantly affects the crystallization kinetics, and the crystallization rate reduces greatly with MW increasing. Moreover, the T-c- and MW-dependent melting behavior of PLLA was clarified with combining the DSC and FTIR results. It was found that the alpha' to alpha-crystalline phase transition occurs prior to the dominant melting in both the low- and high-MW PLLA crystallized at low T-c. Unlike the high-MW PLLA, in low-MW PLLA crystallized at low T-c, the alpha'-form crystals only partially transform into the alpha-one, and some amounts of alpha'-form crystals melt directly without transition during the heating process. With increasing T-c, the melting of PLLA with various MWs changes from the phase transition + melting mechanism to the usual melt-recrystallization mechanism.