Effects of annealing temperature (Ta: 80-140°C) and time (ta: 3-30 h) on the crystalline phase transition in poly(lactic acid) (PLA) were studied by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). In the DSC curves, the sample annealed at Ta=80°C with time interval (ta: 10-30 h) demonstrates a peculiarly small exothermal peak (Texo) around 130°C, just prior to the melting point, corresponding to the disorder-to-order (a’-to- a) phase transition, while the sample annealed at temperature (Ta: 90-110°C) shows a double melting behavior considered as the a'- a phase transition. At Ta>110°C, the a′ -form of PLA was found to transform into the a one during the annealing process. The a’- a phase transition around 100°C (Ta: 90-110°C), determined from the two melting peaks (Tm1 and Tm2) as function of Ta at different times ta, shows a good correlation with the substantial increase in the crystallinity rate (Xc) with a maximum of 32% at Ta=120°C. Towards low temperatures, the glass modulus Eg reported by DMA thermograms, shows an important increase (~30 000 MPa) at Ta=80°C for ta=3 h, due to the nucleation density extremely high in low PLA materials crystallization. After a sharp drop to 3 600 MPa at Ta=110°C, a marked improvement of Eg (15 900 MPa) is observed around Ta=120°C for all samples, regardless of time ta, temperature region where PLA is usually molded in the industrial melt processing. This interesting effect (improvement of Eg in range Ta=100-120°C) can be correlated with the grow of crystallinity in the same domain of Ta, and the a’- a phase transition Ta (Ta: 90-110 °C) determinate by the double Tm melting DSC peak, which is confirmed by the increase of Tg for Ta=90-110°C.