Polyethylene terephthalate (PET) is a widely used polymeric material. In this work, the microstructural features before and after the solid-state polymerization (SSP) of several DuPont PET products were investigated by low-voltage scanning electron microscopy (LV-SEM) and atomic force microscopy (AFM). The microstructural features on the cross section of various PET samples included crystallites, voids, boundaries, defects, and amorphous phases. The SEM images revealed layered and stepped structural features at the micron and 10-micron scales that are highly crystallized at the near-edge region of the cross section for both linear and branched PET samples after the SSP process, The AFM images demonstrate that the degree of crystallization for the linear and branched PET samples increases gradually from the central area to the edge on the cross section. The linear crystallized PET has a higher degree of orientation than the branched crystallized PET in the 10-micron to micron scales, but their crystalline structures have no significant differences in the submicron to nanometer scales, The PET crystallization process occurs when the molecular chains in the amorphous phase are aligned and folded to form straight molecular chains at the nanometer scale, and small crystallites are formed. The crystallites aggregate and align together into a polygon rod-like-shaped crystallites at the submicron scale. Finally, large crystallites at the micron size are formed that appear on the edge area of the cross section.