Abstract This presentation investigates the nucleation of the polyethylene foam extrusion process while using supercritical carbon dioxide as blowing agent. Various percentages of carbon dioxide were used to study expansion ratio, and a fixed percentage with various die opening. Saturation pressure was calculated with each carbon dioxide percentage. Processing pressure was reported for each die opening. Processing pressure drop and saturation pressure drop are addressed as mechanical superheat and chemical superheat, respectively. Either one shows a big variation from experimental distribution in cell density, #/cm3. However, the agreement with experimental results improves significantly when fugacity of carbon dioxide is considered. Although variation still exists, it suggests a critical superheat in foam nucleation, which may help define the threshold for nucleation.