The mixed ion effect (MIE) in Li/Na-NaZr2(PO4)(3) with a three-dimensional diffusion path was observed. The conductivity minimum at an ionic ratio 0.8 was lower by about three orders of magnitude than for either end member. Hexagonal parameters a, c and Li-7 MAS/NMR spectral intensity against the ionic ratio showed a linear change. The spectral width of P-31 MAS NMR at chemical shift 5.9 ppm against the ionic ratio showed a minimum at the same ionic ratio with the conductivity data. The behavior suggests that the conductivity minimum results from the inhomogeneous distribution of two unlike ions and a blocking of diffusion path.