We theoretically investigate the thermoelectric properties of a serial double quantum dot junction system. A two-level Anderson model including electron hoppings and intradot Coulomb interactions as well as interdot Coulomb interactions is employed to simulate the system. The charge and heat currents in the Coulomb blockade regime are calculated by Keldysh Green's function technique. The electrical conductance, Seebeck coefficient, electronic thermal conductance, and figure of merit (ZT) of the system are calculated in the linear response regime. We find that the figure of merit ZT is markedly reduced by the size fluctuation and Coulomb interactions.