Polymeric microcellular foams are used in many industrial and commercial applications where weight is an important factor. Their porous structure can be used to control thermal conductivity, dielectric constant and sound dampening behavior. In addition, the strength-to-weight ratio of structural foams is greater than that of metals. Adding heterogeneous nucleation agents such as inorganic particles into the polymer matrix is very helpful for preparing microcellular foams. Fillers reduce the nucleation free energy activation barrier and force the nucleation to occur at the filler–polymer interface. Because of the lowered activation barrier, higher pore densities (pores/volume) with smaller pore sizes can be achieved. The pore density and size depend on the size of the fillers, filler size distribution, and filler surface chemistry. Nanoparticles offer a potential advantage over micronized fillers because at the same loading (by weight), they offer more nucleation centers and higher interfacial area. In this work Microcellular polystyrene/nano-silica nanocomposites were prepared by Pressure dropping process using supercritical CO2 as the blowing agent in batch process. This study is an effort to control microcellular foam structure with addition of silica nanoparticles in the batch process. The effect of the surface treatment of nanoparticle on the polymer foam was investigated. Silica nanoparticles were modified by reaction with vinyltriethoxysilane. The surface tension of polystyrene, silica and modified silica were determined by contact angle test in order to predict the dispersion of nanoparticles in polystyrene. The effect of surface treatment and nano-silica loading particles on microcellular foaming was investigated by characterizing cell density and cell size of the foamed product using SEM (Scanning Electron Microscope) pictures. The results showed that surface treated nano-silica has substantial effect on decreasing cell size and increasing the cell density.