This study was an effort to control foam structure with controlled shear rate in a slit die with addition of nanosilica in the extrusion process. We used general purpose polystyrene as the matrix, nanosilica as a nucleating agent and nitrogen gas as the blowing agent. For better dispersion of nanoparticles in the matrix, a combination of solution and melt methods were employed. The foams were produced in a twin screw extruder by studying the effects of shear rate on the slit die, size and weight percentage of nanosilica on foam density, cell density and average cell size. Scanning electron microscopicy (SEM) pictures were used to obtain the main characterizing parameters of the foams i.e., cell density and cell size. The results showed that when the shear rate of 8 min-1 on the slit die was increased to 12 min^_-1, due to the breakage of the large cells into several smaller cells, there were observed 61% enhancement in cell density and 18% reduction in average cell size. When the size of nanosilica was decreased from 40 to 12 nm, the cell density was increased from 1.24×10⁸ to 1.73×10⁸ cells/cm3 and average cell size was decreased from 11.5 to 9 micrometer. By changing the nanosilica content from 0.1 to 4% (by weight), it was observed that at 2% (by weight) of nanosilica, the maximum cell density was obtained. In conclusion it was observed that the nanocomposite was produced by 2% (by weight) of 12 nm silica nanoparticles at 12 min^-1 shear rate shows the maximum cell density and minimum cell size.