Extrusion foaming is an important process to engineer and change polymer structure to obtain desired properties. Compared with solid plastic products, plastic foams with high cell density and uniform cell sizes have superior mechanical properties, including toughness, impact strength and improved thermal insulation. Extrusion foaming process includes four major stages: (1) dissolution of a blowing agent into the polymeric phase under an elevated pressure and temperature, (2) nucleation of bubbles via a pressure drop, and (3) growth of bubbles, and (4) stabilization of foam structure. In an extrusion foaming, shear may influence on the foam in two distinct states. On the gas dissolution, shear improves gas distribution by mechanical mixing mechanism. On the other hand, shear change the cell nucleation mechanism and cell density on the final foam. Also, shear can break big bubble to two or more bubbles. However, change the cell size on the foam products. Investigation the shear effect on the cell nucleation and achieving to the smaller and uniform cell size using shear rate are the goals of this work. General purpose polystyrene and supercritical nitrogen was chosen as matrix and blowing agent, respectively. Dissolution of carbon dioxide into the polystyrene was carried out in a twin screw extruder and using a rectangular slit die with different geometries. The effect of the shear rate on the cell density and size was investigated using the SEM (Scanning Electron Microscope) pictures of the foamed samples and the density of them was measured according to ASTM D 2325.