Solid-state foaming of cyclic olefin copolymer (COC, Topas 6017) using carbon dioxide (CO2) was investigated. Before the foaming experiment, the solubility of CO2 in the polymer at various saturation pressure (5-10 MPa) and saturation temperature (40 degrees C) was measured using the ex-situ gravimetric method. The effects of the saturation pressure, foaming temperature, and foaming time on the foam structure were systematically investigated. Ultramicrocelluar foams with a cell size of 0.5 mu m and cell density over 10(12) cells/cm(3) were successfully prepared. Moreover, it was observed that the system, which was strongly plasticized by CO2, would transition from foaming to crazing under certain conditions. The phenomenon was examined in detail and understood by a proposed mechanism that accounts for both homogeneous nucleation and stress fields induced crazing.