Polymer foams have wide application area due to their light weight, resistance to impact, ability of insulation, and damping properties for specific applications. In this study, two different thermoplastic materials, polylactic acid (PLA) and polypropylene (PP), were used to compare their foaming ability in foam injection molding. Chemical foaming agent, azodicarbonamide, was used in foam injection molding and tensile test samples were obtained. Mechanical strength of the polymer foam samples were investigated by tensile test. Scanning electron microscopy was applied in order to observe the differences in cell morphologies of PLA and PP foams. It has been seen that 23.4% higher cell density and 151.2% higher cell size have been seen in the PLA foam when the results were compared with PP foam. This induced sharp reductions in mechanical strength of PLA foam while PP foams gave smooth decreases in the mechanical strength. Weight reduction in PLA was also 366.6% higher than that of PP foam due to its higher cell density.