The effects of plasticizer of on the dynamic mechanical and piezoelectric properties of PVDF and P (VDF-TrFE) films were investigated and their measurements were compared with each other. The addition of plasticizer (tricresyl phosphate) lowered the glass transition temperature of both PVDF and P (VDF-TrEF) films and decreased their moduli. The plasticizer did not affect the crystallinities of PVDF and annealed P(VDF-TrFE), but lowered the crystallinity of unannealed P(VDF-TrFE) significantly. Piezoelectric stress constants (e31) of PVDF and annealed P(VDF-TrFE) increased with the increase in plasticizer content while e31 of unannealed P(VDF-TrFE) decreased because the contribution of the crystallinity decrease was larger than that of plasticizer which made dipole reorientation easier.