Glass-ceramics of the: phosphorus containing Na5RSi4O12-type (R = rare-earth) Na+-superionic conductors (Narpsio-V) were prepared by bias crystallization of glasses with the composition Na4.05Y0.55P0.3Si2.7O9 in an electric field. The conditions for bias crystallization are discussed with respect to the microstructure and the conduction properties. The cross sections which are parallel and perpendicular to the electric field direction showed the ionic conductivities of 0.0923 and 0.132 mS/cm at 300 degreesC, respectively. The microstructure and the electric conductivity of the Narpsio, glass-ceramics perpendicular to the electric field direction were significantly different from those in parallel.