Thermoelectric transport properties (Seebeck coefficient, S, and electrical conductivity, σ) of p -type Bi and Sb tellurides are investigated using a first-principles all-electron density-functional approach. We demonstrate that the carrier concentration, band gap, and lattice constants have an important influence on the temperature behavior of S and that the volume expansion by 5.5% in Sb2Te3 results in an increase in S by 33μV/K at 300 K. We argue that in addition to the electronic structure characteristics, the volume also affects the value of S and hence should be considered as an origin of the experimental observations that S can be enhanced by doping Sb2Te3 with Bi (which has a larger ionic size) in Sb sites or by the deposition of thick Bi2Te3 layers alternating with thinner Sb2Te3 layers in a superlattice, Bi2Te3/ Sb2Te3. We show that the optimal carrier concentration for the best power factor of Bi2Te3 and Sb2 Te3 is approximately 1019 cm-3