  We study dynamical behaviors of the weakly interacting Bose-Einstein
condensate in the one-dimensional optical lattice with an overall double-well
potential by solving the time-dependent Gross-Pitaevskii equation. It is
observed that the double-well potential dominates the dynamics of such a system
even if the lattice depth is several times larger than the height of the
double-well potential. This result suggests that the condensate flows without
resistance in the periodic lattice just like the case of a single particle
moving in periodic potentials. Nevertheless, the effective mass of atoms is
increased, which can be experimentally verified since it is connected to the
Josephson oscillation frequency. Moreover, the periodic lattice enhances the
nonlinearity of the double-well condensate, making the condensate more
"self-trapped" in the $\pi$-mode self-trapping regime.
