Abstract Polycrystalline samples of (Ca 1− x R x ) 3Co 2O 6 with R = Gd, Tb, Dy and Ho at x = 0 – 0.1 were synthesized and the effects of rare earth substitution on their thermoelectric properties were investigated. In the high-temperature region, the rare earth substitution resulted in an increase in the Seebeck coefficients ( S ) , and the S values increased with decreasing ionic radius of rare earth elements in the order Gd 3+>Tb 3+>Dy 3+>Ho 3+. In contrast, the influence of rare earth substitution on the electrical resistivity was small. The high-temperature power factor was thereby improved by the late rare earth substitutions, particularly those with Ho 3+ for Ca 2+. For the Ho-doped samples ( x ≤ 0.05 ) , the power factor was significantly improved by increasing Ho concentration.