The thermoelectric properties and crystallographic features of layered perovskite-type Ca-substituted (Sr1-xCax)3(Ti1-yNby)2O7 (y=0–0.2), Ruddlesden-Popper phases were investigated. The influence of crystal structure and carrier concentration on the carrier effective mass for Ti-based metal oxides based on TiO6 octahedra and having a significant Ca substitution effect are discussed. It was found from structural analysis that the large Seebeck coefficients of Ca-substituted compounds can be attributed to the high values of carrier effective mass, which originates from the enhancement of the symmetry of the TiO6 octahedra. Low thermal conductivities, 3.6–3.8 W m-1 K-1 at 300 K, were obtained for Ca-substituted compositions due to phonon scattering at SrO/(SrTiO3)2 interfaces of the inherent superlattice structure and to mass-defect phonon scattering between Ca (MCa=40) and Sr (MSr=88). The highest ZT value obtained was 0.15 at 1000 K, for 10 at % Ca-substituted Sr3(Ti0.95Nb0.05)2O7.