In comparison to other kinds of crosslinked thermoset foam, recyclable polyolefin foam is an environmentally friendly material, but it increases the degree of foaming in the foam extrusion process. In order to overcome this shortcoming, the effect of material and process variables on the degree of foaming and the growth behavior of the gas bubbles was studied by using polyolefins, such as polypropylene (PP), low density polyethylene (LDPE), and high density polyethylene (HDPE). These materials were foamed in extrusion process with a blowing agent consisting of sodium bicarbonate and citric acid. A higher degree of foaming was obtained with a low temperature of cooling water and high melt temperature. The take-up speed did not influence the degree of foaming. It was also observed from cell morphologies that the size of the gas bubbles was reduced with increased take-up speed and decreased melt temperature, but the size of the gas bubbles was nearly invariant as the temperature of the cooling water increased. In addition, the effects of the molecular weight, the molecular structure, the melt viscosity, and the melt tension on the degree of foaming and the growth of the gas bubbles were investigated. It was revealed that the degree of foaming and the size of the gas bubbles were directly related to the melt tension of the polymer melt, regardless of the structure and the molecular weight of the polymer resin and the temperature of the polymer melt. This implies that the ability to sustain the cell in the melt state is a critical foaming parameter in the extrusion process.