The injection molding of microcellular polymers is expected to be increasable promise for engineering applications. The combined effect of precision mold opening and gas counterpressure process produced uniform microfoam structure with a maximal cell diameter less than 10 μm/1/. The nature of microcellular PP and PC/ABS was analyzed. It was observed that the mechanical properties, the morphology and the application of microcellular PP were influenced by the degree of crystallinity and the conditions of foaming process. The viscoelastic behavior of materials and their correlation between process, foam structure and properties of PP foam and PC/ABS foam were investigated by DMA.