Ultramicrocellular polystyrene (PS) with a cell size range of 100-400-nm and a cell density of 2.03-1012 cells/cm3 has been generated by the facile melt mixing of voided PS nanoparticles and PS without a foaming process. The work was focused on the facile and rapid processing of ultramicrocellular PS using voided PS nanoparticles to overcome the drawbacks of the supercritical gas foaming technique, such as time-consuming batch operations, rigorous processing demands and restricted surface quality. The voided PS nanoparticles were synthesised via a one-step emulsion polymerisation and acted as pore makers in ultramicrocellular processing. The results showed that this new method might be superior to supercritical gas foaming to produce ultramicrocellular or microcellular polymers, since it has simple processing and relatively accessible processing conditions. Thermal analysis indicated that this strategy could be useful to prevent surface swirl marks and thus represents a major step towards a broad range of applications of ultramicrocellular and microcellular polymers.