A new series of oxides NaCo2-xMxO4 (M = Mn, 0.0 less than or equal to x less than or equal to 1.0; M = Ru, 0.0 less than or equal to x less than or equal to 0.5) were synthesized, and their crystal structure, electrical conductivity, thermoelectric power, and magnetic properties were studied. All the samples crystallized in hexagonal systems. The electrical resistivity and the Seebeck coefficient of NaCo2-xMxO4 (M = Mn, Ru) increase with increasing Mn content. The magnetic properties of NaCo2-xMnxO4 change with increasing Mn content.