The ionic conductivity of Li3PO4 dispersed Li0.5La0.5TiO3 (LLTO) composite solid electrolyte was examined by using complex impedance analysis. The X-ray powder diffraction (XRD), scanning electron microscope (SEM) observation and relative density measurement were also performed to study the dispersion of the second phase into the LLTO electrolyte. It was found that the addition of Li3PO4 into LLTO matrix leads to a decrease of the ionic conductivity, though the composites added by the Li3PO4 particles show a high compactness. The reasons for the conductivity deterioration were clarified based on the lattice structure of LLTO, misconstrues at the grain boundary regions, and the lithium content. The variation of the lithium content and microstructures in the LLTO matrix dispersed by Li3PO4 particles are mostly responsible for the behaviors in the ionic conduction.