Abstract: Thermoelectric properties of Al substituted compounds Ca₃(Co_1-xAl_x)₄O9 (x = 0, 0.03, 0.05), prepared by a sol-gel process, have been investigated in the temperature range 305–20 K. The results indicate that after Al substitution for Co in Ca₃(Co_1-xAl_x)₄O₉, the direct current electrical resistivity and thermopower increase due to the reduction of carrier concentration. Experiments show that Al substitution results in decreased lattice thermal conductivity. The figure of merit of temperature behavior suggests that Ca₃(Co_0.97Al_0.03)₄O₉ would be a promising candidate thermoelectric material for high-temperature thermoelectric application.