<p>Thermally expandable microcapsules (MC) were studied as foam injection molding agent for mold-back process and short-shot process. Various types of polypropylene (PP) were adopted for base polymer. Expansion of MC was evaluated from analysis of scanning electron microscope images. The relationship between shear viscosity of PP and expandability of MC was studied. The expanded diameter of MC decreased as shear viscosity increased. It was concluded that shear viscosity is an appropriate parameter for examining MC expandability in the foam injection process. For the case of mold-back process, it was found that the diameter of MC in the center region of specimen does not depend on the type of PP.</p>