In this study, microcellular polypropylene (PP) electrets were fabricated under various charge and process conditions. This was followed by thermally stimulated current (TSC) experiments to examine in a qualitative manner the charge stability of the electrets and the origin of the surface and space charges. In addition, a series of experiments was performed to obtain the effective piezoelectric constant in the thickness direction using an equation that considers external inertial loading effects. An optimization process using the Taguchi method was conducted to find the optimal conditions for the charging process and the electret properties that maximize the piezoelectric effects. To compare the contributions of signal parameters, a pooled analysis of variance (pooled ANOVA) was used. From these results, the factors that most significantly influence the piezoelectric effects were identified. A response surface was constructed to predict the optimal values of these factors to obtain the best piezoelectric effects. As a consequence, the applied voltage and the modified film thickness were turned out to be the most influential factors, and it was predicted that the optimal conditions are a 30-kV corona discharge and use of $60-{mu}m$-thick film.