Epoxy resins (bisphenol A type epoxy resins/2-ethyl-4-methylimidazole) consisting of oligomers with different molecular weights were foamed using a temperature-quench physical foaming method with CO2. The resulting cell morphologies could be classified into four types: non-foamed structure, cracked structure, star-shaped structure, and sphere-shaped structure. The effects of the gel fraction and molecular weight between crosslinks (MC) on the cell morphology were investigated for the preparation of microcellular epoxy foams. MC was calculated by measuring the plateau rubber modulus of the rheological properties and the weight uptake of acetone. By varying the molecular weight of the epoxy oligomers and the cure time, the MC of the epoxy was controlled to modulate the cell morphology. The experiments elucidated the threshold MC value that permits CO2-bubble nucleation: CO2-bubble nucleation in the epoxy resin could be induced when the distance between the crosslinking points exceeded the critical size of bubble nucleus. Based on this information, the microcellular epoxy foam was prepared by maintaining MC above 104g mol−1 and the complex modulus above 6 × 108 Pa. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40407.