Structure refinement and electrical conductivity measurement were performed for a series of spinel oxides, (1-x)Li4/3Ti5/3O4-xLiCrTiO(4). The AC conductivity with composition x was abruptly increased for x>0.6, but no structural phase transition was obtained from structure refinement data by Rietveld analysis. Since the shape of complex impedance plots changes at x=0.6, it is assumed that mechanism of electrical conduction changes at the composition. From the site-percolation theory conduction pathways by d electrons of Cr3- in octahedral site are connected at x=0.623. Accordingly it may be concluded that there is only ionic conduction in x<0.623, while mixed conduction by both ions and electrons exists in x greater than or equal to 0.623.