In order to explore the piezoelectric mechanism of the poly(vinylidene fluoride) (PVDF) film, the true piezoelectric constant was expressed in terms of a combination of measurable mechanical and piezoelectric constant values. Each piezoelectric constant was divided into four subgroups according to their boundary conditions and then their interrelations were derived. In addition, the values of piezoelectric constants by the dimensional effect only were calculated. Thus, if we determine the ten constants of piezoelectric film, we can reveal the contribution of the dimensional effect in the piezoelectric mechanism.