Foams from poly(phthalazinone ether sulfone ketone) containing biphenyl moieties (PPBESK) are prepared by a two-stage batch process using CO2, as foaming agent. Closed microcellular and bicontinuous nanoporous cell morphologies are observed. The effects of saturation pressure, the concentration of CO2, dissolved in PPBESK, and foaming temperature on the porous structure were studied. The results indicate that the key factor for the structure transformation is the concentration of CO2, dissolved in PPBESK. When the CO2, concentration is above the critical value (2.80 g CO2/100 g polymer), the dosed microcellular structure changes to bicontinuous nanoporous. So controllable preparation of closed microcellular and bicontinuous nanoporous foams from PPBESK can be achieved by controlling the concentration of CO2, which dissolved in the polymer. Moreover, the tensile properties of the foams were investigated. It is found that the tensile strength of the foams is dosely related to its cell structure especially the cell type.