Thermoelectric properties of Al substituted compounds Ca3(Co1?xAlx)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(Co1?xAlx)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(Co0.97Al0.03)4O9 would be a promising candidate thermoelectric material for high-temperature thermoelectric application.