Poly(lactic acid)/poly(trimethylene carbonate) (PLA/PTMC) films incorporated with cinnamaldehyde (0, 3, 6, 9, and 12 wt%) were prepared by solvent casting and characterized by physical, mechanical, and antimicrobial properties. SEM analysis revealed that the surface of film became rougher with certain porosity when cinnamaldehyde was incorporated into the PLA/PTMC blends. Cinnamaldehyde acted as plasticizers which reduce the intermolecular forces of polymer chains, thus improving the flexibility and extensibility of the films. Differential scanning calorimetry showed that the crystallinity of PLA phase decreased by the presence of cinnamaldehyde in the composite films. Water vapor permeability of films increased with the increase of cinnamaldehyde loading. However, the active PLA/PTMC/cinnamaldehyde composite films showed adequate barrier properties for food packaging application. Incorporation of cinnamaldehyde to the PLA/PTMC polymer matrix improved the antimicrobial activity of active packaging films. These results indicated that the best compromise between mechanical, barrier, thermal, and antimicrobial properties could be achieved by the addition of 9 wt% cinnamaldehyde into PLA/PTMC blends.