Na-ion conducting Na1+x[SnxGe2-x(PO4)(3)] (x = 0, 0.25, 0.5, and 0.75 mol%) glass samples with NASICON-type phase were synthesized by the melt quenching method and glass-ceramics were formed by heat treating the precursor glasses at their crystallization temperatures. XRD traces exhibit formation of most stable crystalline phase NaGe2(PO4)(3) (ICSD-164019) with trigonal structure. Structural illustration of sodium germanium phosphate [NaGe2(PO4)(3)] displays that each germanium is surrounded by 6 oxygen atom showing octahedral symmetry (GeO6) and phosphorous with 4 oxygen atoms showing tetrahedral symmetry (PO4). The highest bulk Na+ ion conductivities and lowest activation energy for conduction were achieved to be 8.39 x 10(-05) S/cm and 0.52 eV for the optimum substitution levels (x = 0.5 mol%, Na-1.5[Sn0.5Ge1.5(PO4)(3)]) of tetrahedral Ge4+ ions by Sn4+ on Na-Ge-P network. CV studies of the best conducting Na-1.5[Sn0.5Ge1.5(PO4)(3)] glass-ceramic electrolyte possesses a wide electrochemical window of 6 V. The structural and EIS studies of these glass-ceramic electrolyte samples were monitored in light of the substitution of Ge by its larger homologue Sn.