Polyethylene terephthalate (PET) foams were prepared at different saturation temperatures using two supercritical foaming methods. The average cell size, cell number density, and porosity of PET foams obtained using each foaming method were compared. The crystallinity of the PET samples after the saturation step in the two-step foaming process was measured to observe the CO2-induced crystallization. The crystallinity of PET according to the saturation temperature led to a variation in cell size in the two-step foaming. In contrast, the melting of crystals with the increase in the temperature affected the cell number density of the polymeric foam prepared by one-step foaming method. The influence of the PET crystals on the cell nucleation or cell growth in each foaming method was studied from these results.