The interfacial diffusion reaction between lead zirconate titanate (PZT) and lead nickel niobate (PbNi1/3Nb2/3O3:PNN) phases in the PZT/PNN functionally gradient piezoelectric ceramics were investigated as a function of the diffusion temperature and time, respectively. The ionic composition distribution profiles in the interdiffusion region were examined by electron probe microbeam analysis (EPMA). Based on a diffusion model of the overlapped diffusion solution from thin slab, the numerical simulation for the ionic composition distribution was carried out by computer, which was in agreement with the EPMA result. The diffusion coefficients for the Ni2+, Nb5+, Ti4+ and Zr4+ ions were determined, which were 33.8, 22.6, 10.8 and 9.9 x 10(-12) m(2) s(-1), respectively. The apparent activation energies for these ions were 94.4, 171.7, 257.5 and 325.8 kJ mol(-1), respectively. The differences in the ionic diffusion coefficients and apparent activation energies were discussed from the viewpoint of the crystal chemistry.