In this paper, the foaming process and final microstructure study of polystyrene/ polymethyl methacrylate alloy foams were studied using normal pentane as a foaming agent at constant pressure. For this purpose, PS / PMMA blend was prepared in different percentages with /without calcium carbonate nano. In order to investigate the foaming process, the polymer sheets, 200 μm thick, were saturated with normal pentane gas and studied in the foaming process with sudden drop in pressure. The results showed that with increasing temperature the saturation process was faster and with the increase of PMMA in the blend due to higher melt strength the gas penetration into the polymer decreased and the foam density decreased. The nanoparticles resulted in an increase in the nucleation rate and numerical density of the bubbles during foaming as well as the size of the final foam structure.