The co-substituted Ca1-xGdxMn1-xNbxO3 (0 <= x <= 0.1) perovskite are synthesized by solid state reaction. Rietveld refinement analysis of powder XRD confirms orthorhombic structure (Pnma) for all the compositions. The lattice parameters and cell volume increase gradually with dopant concentration. The thermopower and electrical resistivity (x <= 0.08) decrease with increase of dopant concentration i.e. with increasing the Mn3+ ions having e(g)(1) electrons. At higher dopant concentration (x > 0.08) the resistivity starts increasing due to the reduced mobility of charge carriers. The enhanced octahedral distortion in the crystal lattice of co-substituted Ca1-xGdxMn1-xNbxO3 is responsible for the reduction of charge carrier mobility. The transport mechanism of charge carriers are explained in the framework of Mott's small polaron hopping mechanism. The highest power factor of 225 mu W m(-1) K-2 and ZT of 0.15 are obtained at x = 0.04 composition at 800 K. (C) 2014 Elsevier B.V. All rights reserved.