Highly dense CaMn1-xMxO3-δ (with M = Nb, Mo, Ta, W and 0 ≤ x ≤ 0.08) n-type thermoelectric materials with low electrical resistivities are prepared from nano-crystalline powders. Their room temperature power factors outperform the best reported results by 30% or more. In combination with the thermal conductivities promising figure-of-merits of ZTM=Ta,x=0.04 = 0.21 and ZTM=W,x=0.04 = 0.20 were achieved at 1160 K. The relative changes and temperature dependencies of the Seebeck coefficient, the electrical resistivity, and the power factor are described with a small-polaronhopping-based mechanism. In the limits of high-temperatures and low substitution levels the Seebeck coefficients are in good agreement with Heikes formula. At high substitutions the efficiency of the doping presumably decreases due to trapping states caused by the formation of bands from Jahn-Teller lowered eg orbitals of Mn3+. Jahn-Teller distortion of Mn3+ also leaves its footprints in the orthorhombic distortion of the crystal structure along the b-axis.