Thermoelectric properties of metal/molecule/metal junction is investigated for different lengths of polythiophene connected to three dimensional metallic electrodes, using Green function formalism in linear response regime. An extended Su-Schrieffer-Heeger model is used to describe the Hamiltonian of polythiophene molecule and a tight-binding model is chosen to study the effect of length change on thermoelectric coefficients of the molecular junction. Increasing the molecular length results in decrease of HOMO-LUMO gap, decrease of the electrical conductance and increase of the value and oscillation of the thermopower. In addition, our results show that the temperature dependence of the thermoelectric coefficients is strongly related to the molecule length. (C) 2014 Elsevier B. V. All rights reserved.