Abstract: The isothermal crystallization kinetics of polylactic acid (PLA) was investigated by differential scanning calorimeter (DSC), and two initial states, polymer melt and glass state, were concerned particularly. The influences of initial states of PLA and various temperatures on the crystallization behavior were discussed. When PLA is isothermally crystallized from glass state, activation energy (359 kJ/mol) is higher than that from polymer melt (103 kJ/mol). Exponent of Avrami, n, is just reverse; the former is 2.3—2.7 and the latter is 2.5—3.1. There is no appreciable distinction between the equilibrium melting points: respectively 156.4 ℃ from polymer melt and 155.7 ℃ from another. From glass state, induced time decreases gradually with the increasing of crystallization temperature. When crystallized from polymer melt, the shortest induced time (9.47 min) is observed at 95 ℃. Furthermore, the control steps of PLA crystallization rate, in the whole temperature range, are different owing to their initial states. Initiated from the glass state, the rate depends strongly on crystal growth process. While initiated from the melting state, the rate is controlled by the foregoing rule at low temperature, and by nucleation process at high temperature.