Ultrahigh-cell-density polypropylene (PP) foam was prepared by supercritical carbon dioxide (scCO(2)) foaming in the presence of porous polytetrafluoroethylene (PTFE) micropowder. The voids of PTFE microparticles were filled with supercritical CO2, causing them to split into multiple granules due to the force of its expansion during the pressure release, resulting in the formation of a large number of nucleation sites. The cell density of this foam reached 10(10)-10(11) cells/cm(3), which was 2-5 orders of magnitude higher than pristine PP foam. We proposed that the nanoscale granules resulting from the splitting of PTFE microparticles, and the growth of nucleated small cells that generated the local strain field variation in the multiple-phase system, were responsible for the considerably increased nucleation number of PP foaming. Additionally, the tensile strength and sound absorption property of PP/PTFE foam were largely enhanced and the preparation of ultrahigh-cell-density PP/PTFE foam was easily controlled over a wider foaming temperature window.