Based on the formula of Na(3+3x-y)R(1-x)P(y)Si(3-y)O(9), Na5YSi4O12-type glass-ceramic Na+-fast ionic conductors were successfully produced for a variety of rare-earth ions, R, under the appropriate composition parameters. The possible combinations of x and y became more limited for the crystallization of the fast ionic conducting phase as the ionic radius of R increased, while the Na+ conduction properties were more enhanced in the glass-ceramics of larger R. These results are discussed in view of the structure and the conduction mechanism. Also studied were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. These effects were understood in relation to the grain boundary conduction properties as well as the transmission electron microstructural morphology of grain boundaries.