Nano-cellular materials can be defined as porous materials with an average cell size lower than 1 μm. They are widely investigated for many applications such as thermal insulation or dielectric applications. However, their fabrication still remains tough. CO2 foaming process is already used to create polymer micro-foams [1]. It also may prove to be a relatively simple way and an environment-friendly technique to create nano-cellular materials. The gaseous CO2 batch process investigated here is divided into two steps. First, the compressed gas dissolves in polymers until saturation into a pressure vessel. The samples are then heated by transferring them to a hot water bath to induce the foaming. Nano-foams formation requires a high nucleation rate and the limitation of the cell growth during the second step. For that purpose, the control of the foaming is essential and is going to depend on process pressures and temperatures but also on the polymer material microstructure. One way to increase the nucleation rate is to create a heterogeneous nucleation inside a polymer/nucleating agents system, while controlling the foaming process parameters. This study focuses on the foaming of PMMA/MAM (poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) triblock copolymer blends, whose the heterogeneous morphology with dispersed nodules of soft phase would improve the nucleation rate [2]. The investigated foaming process is a two stages batch process using a low saturation pressure. The influence of the foaming process parameters and the initial microstructure of the blends on their final foam structures are presented. Under certain conditions, nano-cellular materials with an average cell size of 100 nm are obtained, a two decades decreasing compared to that of neat PMMA foam. [1]E. Reverchon, S. Cardeas, J. Supercrit. Fluids. 2007, 40, 144-152 [2]J.A.R. Reglero Ruiz, M. Dumon, J. Pinto, M.A. Rodriguez-Perez, Macromol. Mat. Eng. 2011, 296 752-759