The effect of electron-phonon interaction (EPI) on the thermoelectric properties of a double quantum dot system weakly coupled to external electrodes is analyzed using master equation formalism. The influence of the EPI, temperature, interdot tunneling strength, and Coulomb repulsions on the electrical and thermal conductances, thermopower, and figure of merit is studied in detail. The electrical conductance is reduced in the presence of the EPI, while the thermal conductance is increased. Furthermore, it is observed that the magnitude of the electrical and thermal conductances can be tuned by using the interdot tunneling strength. It is also found that although the magnitude of the figure of merit is reduced in the presence of the EPI, the number of peaks is enhanced. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767376]