Solid solution NayK1-yNa1.4Zr1.3Mg0.7(PO4)(3) was prepared between KNa1.4Zr1.3Mg0.7(PO4)(3) and Na2.4Zr1.3Mg0.7(PO4)(3) with NASICON type structure. Cationic substitution improved the conductivity which reached a maximum for y is approximately to 0.7. For each compositions, the activation energy was obtained from the Arrhenius plots, and showed fairly constant against y. Hexagonal parameters a and c were obtained by X-ray diffraction. The a parameter was fairly constant with y, while the c parameter had a maximum for y is approximately to 0.7. The p value, which means non-Debye parameter, shows a nonlinear behaviour against y. These results are correlated with structural features and conductivity data.