Al substituted Li 1+x Al x Hf 2−x (PO 4 ) 3 (x=0.0∼0.5) solid electrolytes were prepared by conventional sintering (CS) and spark plasma sintering (SPS), and the Li + ion conductivity of the sintered pellets was examined using an impedance analyzer. All the samples exhibited a rhombohedral NASICON-type structure. SPS improved the densification compared to CS, and Al substitution further increased the sample density resulting in a maximum density of ∼90%. The Li + ion conductivity increased in proportion to the sample density and the highest conductivity of 1.1×10 −4 Scm −1 was found in the Li 1.5 Al 0.5 Hf 1.5 (PO 4 ) 3 SPS specimen, which is comparable to the best value reported elsewhere. The activation energy of the total conductivity (bulk+grain boundary) reduced from 0.39 to 0.28 eV with Al substitution. The enhanced conductivity was discussed in terms of the densification, bottleneck size, and lithium ion content.