Semi-crystalline polymer like poly(L-lactic acid) (PLLA) is generally regarded as a multi-phase system where crystalline domains disperse in the amorphous matrix. The crystallization rate of PLLA is very slow under atmosphere. However, the crystallization process can be accelerated under compressed CO2, because the dissolved CO2 induces the swelling of polymers and decreases the energy barrier for the molecular retraction and fold. Furthermore, the intense interaction between the electron-donating carbonyl oxygen of PLLA and the electron-accepting carbon of CO2 greatly promotes the CO2 dissolution and then reinforces the plasticization effect. In the present work, CO2-induced spherulites were successfully utilized to regulate the morphology and structure of poly(L-lactide) (PLLA) foam with cells size ranging from nanoscale to microscale. The unique transitions from nanocells to microcells through need-like cells and bimodal cells in the spherulites formed under CO2 were investigated in detail. According to the characteristics of the DSC curves and the cell morphology, the saturation temperature was divided in three regions: Regions I (60-80 oC), II (90-110 oC) and III (115-120 oC). Results showed that cells could nucleate and growth in between the interlamellar amorphous layers in the spherulites formed under CO2. It was concluded that the crystallization morphology of PLLA treated under CO2 played a significant role in the cell nucleation and growth.