We report a novel approach to prepare injection-molded foams using air as the foaming agent, which is realized through our newly developed foam injection molding technology. Microcellular polypropylene (PP) foams exhibiting cell size of similar to 7 mu m and cell density of 1.63 x 10(9) cells/cm(3) were successfully obtained employing air foaming. Fourier transform infrared spectroscopy and theological results illustrated that the molecular structures and melt properties of PP were greatly altered in air foaming, resembling a reactive foaming process. Furthermore, compared with nitrogen and carbon dioxide foaming, air foaming gave us a more flexible PP foam with finer cellular structure.