The microvoid formation process that takes place during the plastic deformation of beta-form polypropylene (PP) was investigated at different crystallization and drawing temperatures. It was found that the porosity increased with both a higher crystallization temperature and a lower drawing temperature. This fact could be understood in terms of the stability of the beta crystals of the original film. The more stable beta crystals that formed at a higher crystallization temperature had a lower molecular chain mobility and thus hindered the bulk contraction of the film, while at the same time the volume contraction caused by either a beta-alpha or a beta-smectic transformation inside the film enlarged the microvoids at the end, or between the microfibrils.