Sodium superionic conductors with the composition of Na5.20Y0.80P0.40Si3.60O12.0 were prepared using highly homogeneous powders synthesized by reactions of alkoxides. The single phase specimen with highly ionic conductivity of rho300=9.47 OMEGA cm at 300-degrees-C was obtained by sintering in N2. The grain sizes of samples obtained by using metal alkoxides were 2-3 mum and the grain growth was promoted. The addition of calcia improved sinterability.