Solid electrolytes, LiTi2(PO4)(3) (LTP), Li1.3Al0.3Ti1.7(PO4)(3) (LATP), and Li1.3Al0.3Ti1.7(PO4)(2.9)(VO4)(0.1) (LATPV), were prepared by conventional sintering (CS) and spark plasma sintering (SPS) methods, and the Li+ ion conductivity of the sintered pellets was examined using an impedance analyzer. SPS remarkably improved the densification compared to CS and resulted in dense ceramics (95-97% of theoretical density) irrespective of the substituted ions. The highest conductivity of 2.6 x 10(-4) S/cm was found for the LATPV specimen sintered by spark plasma at 1100 degrees C. LATP and LATPV exhibited an order of magnitude higher ionic conductivity than LTP in the specimens of similar densities. The results demonstrated that the enhanced conductivity in substituted LTP is not due to the enhanced densification alone. The other possible explanations are discussed in terms of bottleneck size, lithium content, and grain boundary characteristics.