Polymer foams of various cell sizes within the microcellular range with relative densities ranging from 0.86 to 0.03 were prepared by a solid-state foaming where supercritical carbon dioxide (ScCO2) was used a physical blowing agent. Experiments were carried out on these foams to investigate the relationship between their cell morphologies and mechanical properties (tensile and impact).The foams produced were characterized by scanning electron microscopy (SEM) to study the cell structures，cell morphologies and cell distributions. Differential scanning calorimetry (DSC) was use to obtain the thermal properties and the crystallinity whiles x-ray diffraction (XRD) was use to investigate the crystal structure. The Izod notched impact test was use to acquire the impact data and the tensile data by the standard test for acquiring tensile strength，tensile modulus and elongation.The tensile moduli of the foams were found to be lower than that of the un-foamed counterparts in all cases but contrary to that，other properties namely tensile at break，elongation and impact strength，they were found to have improved as per the relative densities and cell sizes of the foams. Foams with cell size below 3μm showed higher tensile at break than the un-foamed samples. Similarly，foams with cell sizes less than 10μm were also found to have higher elongation at break and impact strength compared to the un-foamed ones.A study of the impact surfaces of the polymer foams with cell sizes below 10μm showed an obvious plastic deformation which confirmed the relevance of size reduc-tion and collapse of cells in the absorption of impact energy. This was evident from striations perpendicular to the direction of the impact that appears on the fractured surface. It also showed that when cell size increased he impact energy is distributed over the whole surface area and as such there are no different zones on the fractured surface.The introduction of other materials into the polypropylene in the form of voids and other polymeric materials altered the mechanical properties. Different compositions of PP and EPDM were used to investigate the effect of material blend composition had on the mechanical properties of the subsequent foams. For a PP/EPDM foam atthe specified processing conditions it was found that composition 10％by weight of EPDM was the best of optimum mechanical performance although the improvement in the mechanical properties were not significantly high as anticipated.