A scanning electron microscope technique is employed to investigate the charging response of stretched semi-crystalline polypropylene (PP) and high density polyethylene (HDPE). The results reveal that the charging response of these insulating polymers varies with the amount of deformation due to stretching. The curves of charging response as a function of extension for HDPE and PP both can be divided into four regions showing up, down, and up and down trends, respectively. These four regions are related to four different deformation stages in the semicrystalline polymers, and polaron formation appears to be appropriate for the interpretation of the charging behavior.