Polycrystalline samples of (Na 1 - y M y ) x Co 2 O 4 (M=K, Sr, Y, Nd, Sm and Yb; y=0.01~0.35) were prepared by a solid state reaction method. In this study, in order to improve the thermoelectric properties of Na x Co 2 O 4 , the effects of partial substitution of other metals for Na on the thermoelectric properties of Na x Co 2 O 4 from room temperature to 1073 K were investigated. For M=Sr, the thermoelectric power and the electrical resistivity increased, and the electronic and lattice contribution to the thermal conductivity decreased compared to the non-substituted sample. These effects suggest that the carrier density was reduced by the substitution of Sr for Na. As a result, the figure of merit of the sample for M=Sr was improved. On the other hand, for other samples in spite of the increase in the electrical resistivity, the thermoelectric power decreased. These results are anomalous effects, which cannot be described merely by a change of the carrier density. For all samples, except for M=Y, the lattice contribution to the thermal conductivity decreased and for all samples, except for M=K, the electronic contribution slightly decreased.