Microcellular isotactic polypropylene (PP) foams with various cell sizes (1-50 mu m) and relative densities (0.86-0.04) were prepared using supercritical carbon dioxide (CO2) solid-state foaming to investigate the relationship between the cell morphologies and mechanical properties (tensile and impact). The tensile modulus of the PP foams decreased proportionally with the square of their relative densities and was always smaller than that of the un-foamed PP. Contrary to the tensile modulus, other properties, namely, tensile strength at break, elongation at break and impact strength of the PP foams, outperformed the un-foamed PP with margins that depended on the relative densities and cell sizes of the foams. PP foams with cell sizes less than 3 mu m showed higher tensile strength at break than the un-foamed PP. During tensile deformation, cells stretched from circular shapes to elliptical ones. They collapsed or broke at very high deformation. Small cells (less than 3 mu m) were found to significantly reduce the stress concentration under loading. Additionally, PP foams with cell sizes less than 10 mu m showed higher elongation at break and impact strength compared with that of the un-foamed PP. During the impact, the cell size and cell density both decreased gradually along the impact direction. Furthermore, a plastic deformation zone occurred when the cell size was less than 10 mu m, indicating that size reduction and collapse of cells could absorb a significant amount of impact energy. (C) 2016 Elsevier B.V. All rights reserved.