A functionally gradient piezoelectric actuator based on PbNi1/3Nb2/3O3-PbZrO3-PbTiO3 (PNN-PZ-PT) system was prepared. The cantilevered actuator consists of a high piezoelectric & low dielectric layer, a low piezoelectric & high dielectric layer and a thin middle diffusion layer. Frequency dependences of impedance were measured from 100Hz to 3MHz. The resonance modes were characterized. A number of low frequency modes were observed and identified as bending resonance. The dimension dependence of resonance frequency of bending resonance was investigated. The results of the measurement were compared with calculated values. There is a good agreement between the determined resonance frequencies and calculated values.