Polycrystalline samples of (Ca1−xRx)3Co2O6 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 Gd3+ > Tb3+ > Dy3+ > Ho3+. 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 Ho3+ for Ca2+. For the Ho-doped samples (x 0.05), the power factor was significantly improved by increasing Ho concentration.