Polymethacrylimide (PMI) foam is considered as a kind of high performance foam with the highest mechanical strength and is extensively used as the high-performance sandwich composites in the high-technology realm, such as aviation and space flight. In this study, we examine supercritical foaming in compare with chemical foaming method. The monomers used in this research were Acrylonitrile (AN), Methacrylic acid (MAA) and Acrylamide (AM). Azobisisobutyronitrile (AIBN) was used as the thermal-sensitive initiator. 1-Amylalcohol was used as the foaming agent. The materials were mixed and dissolved together and then copolymerized via the mode of bulk casting in the water bath 48-56 °C temperature and atmosphere pressure. Expandable copolymer boards were foamed by chemical foaming as the boards were put in an oven with the 180 °C temperature for 20-30 min. For supercritical foaming the samples were put in high pressure tank (80 psi) about 24 hours to saturate of Co2. Then were taken out and thrown in 3 different hot oil plates with 160, 170 and 180°C temperatures to expand, Then were cooled after 1, 2, 3, 4, 5, 6 and 7 minutes of expanding to achieve different densities. FTIR curves of samples in three different periods of reaction show the reaction process and progress. Super critical foaming is easier to control and more practical as regards chemical foaming is very thermal sensitive and producing the uniform foam is more difficult. In addition as SEM images show cells by supercritical foaming are smaller and more smooth but percent of closed cells is less because of higher gas pressure. SEM images used to compare both ways with about 0.05 gr/cm3 and 0.07 gr/cm3 densities. Cross linking occur after Foaming operation of PMI terpolymer. Higher mechanical and thermal properties are predicted for super critical foaming because of more number of cells. Super critical foaming is more manageable and easier to control. The produced foam is more uniform because in chemical foaming distributing the foaming agent is very difficult and sensitive but co2 diffuse in terpolymer very uniformly.