In situ reflection-absorption infrared (RAIR) spectroscopy was used to investigate the molecular orientation and crystallization dynamics of PLLA thin films crystallized at various temperatures. The results show that the annealing temperature has significant effect on the lamellar orientation of PLLA thin films, which is confirmed by atomic force microscopy (AFM) data. It is found that edge-on lamellar crystals of PLLA thin films are mainly formed by cold crystallization at low temperatures (70-90 degrees C), whereas the flat-on crystals occur when the crystallization temperature reaches to 100 degrees C. Of particular note, our in situ RAIR. data collected during the whole isothermal crystallization process suggests that the orientation of PLLA chain takes place in the early stage of the crystallization process. Moreover, the analysis on the crystallization dynamics of PLLA thin film indicates that the crystal growth dimensional and nucleation modes are also strongly affected by the crystallization temperature.