The role of mixed-valence structure in colossal dielectric constant (CDC) behavior has been investigated in LaFeO3 ceramics by tuning the ratio of Fe2+/Fe3+ through substituting Al for Fe. The ratio of Fe2+/Fe3+ is decreased gradually from 1.0 to 0.0 by increasing the concentration of Al3+. Two clear-cut correlations have been found: (i) the relationship between the CDC behavior and the ratio of Fe2+/Fe3+ follows an exponential function and (ii) the activation energy of the polaron relaxation is proportional to 1/epsilon 02, where epsilon 0 is the intrinsic dielectric constant. These findings underscore the role of the mixed-valence structure in CDC behavior and suggest that adjusting the mixed-valence structure through doping/alloying can be a promising strategy to achieve superior CDC behavior in transition-metal oxides.