In this study, poly(lactic acid) (PLA) resins with linear (L-PLA) and branched structure (B-PLA) were selected, and the solid state foaming technology was applied to prepare PLA foams. B-PLA foams exhibited a high expansion ratio of about 40 and cell density of 10^5–6 cells/cm³, whereas L-PLA foams only had the highest expansion ratio of 29.8 and cell density of 10^3–6 cells/cm³. When PLA resins were induced crystallization during CO₂ saturation, however, the prepared L-PLA foams presented the highest expansion ratio of 37.4 and cell density of 10^6–7 cells/cm³. The cell structure evolution of PLA foams with the foaming time suggested that the in situ formed crystal domains supplied nucleating sites to enhance cell nucleation and acted as physical cross-linking points to stabilize cell structure. These interesting results demonstrated that the induced crystallization might be more attractive than the chain modification to improve the foaming behavior using solid state foaming technology.