Piezoelectric ceramics with large energy density coefficient d(33)g(33) value have been found suitable for piezoelectric energy harvesting applications. In this study, the phase structures and piezoelectric properties of xPb(Zr0.5Ti0.5)O-3-yPb(Zn1/3Nb2/3)O-3-(1-x-y)Pb(Ni1/3Nb2/3)O-3 (xPZT-yPZN-(1-x-y)PNN) ceramic were investigated with systematically varying PZN and PNN components. The ternary phase diagram of PZT-PZN-PNN system was illustrated and the composition region of morphotropic phase boundary (MPB) was determined. Piezoelectric and dielectric measurements verify that the materials in MPB region all present large d(33) and d(33)g(33) values. In particular, very high d(33)g(33) coefficients of 20162.2x10(-15)m(2)/N and 21026.3x10(-15)m(2)/N are observed from samples 0.75PZT-0.15PZN-0.1PNN and 0.8PZT-0.05PZN-0.15PNN with compositions located on the rhombohedral phase side near MPB because the dielectric coefficient epsilon(T)(33)/epsilon(0) decreases faster than the d(33) coefficient at this side.