Polymer foams are used for thermal insulation and weight lightening in various fields such as buildings, automobiles, and LNG containment systems. Phenolic foams are preferred for thermal insulation due to its lower flammability and lower gas generation than any other polymeric insulation foams. In case of the conventional foaming methods, however, it takes long time to fabricate large volume and the environmental regulation limits the use of blowing agents. In this study, new foaming methods for the activated carbon (AC) reinforced microcellular phenolic foams was developed using microwave instead of the blowing agents, and its thermal and mechanical properties were measured with respect to the foaming conditions such as aging time of the resin before microwave foaming and weight percent of the AC. From the results, it was found that the microcellular phenolic foams fabricated under optimum foaming condition had low thermal conductivity and low density suitable for insulating foams.