This article discusses the formation of extruded thermoplastic foam sheet due to expansion of supersaturated physical blowing agent while it exists from a high-pressure flow region into atmospheric surroundings. Swarms of tiny gas cavities of unstable nature are formed and gas diffusion virtually promotes expansion, the rate of which has a strong dependency upon degree of supersaturation. However, foam sheet cooling causes increase of rheological properties to prolong the sheet formation period, during which gas molecules close to the sheet surface tend to migrate onto the surface and evaporate away from the surface to reduce foaming efficiency. The viscoelastic ＇＇cell＇＇ growth model was modified to incorporate the gas loss from the sheet surface. Foaming efficiency appears to be dependent upon blowing agent/resin ratio, foam thickness, and nucleation density (#/cc). The simulation results predicted by the model show a good agreement with experimental data for polyethylene foam with low-percentage difluorochloromethane (HCFC-22) blowing agent.