We found that the dispersive properties of impurity bands in a coupled-resonator optical waveguide based on photonic crystals depend on the properties of the fields localized at the two neighboring defects instead of the distance of the defects. In fact, the seemingly anomalous dispersion of the impurity band in the first Brillouin zone is a result from band folding. After calculating the effective refractive index of the structure, we have verified that the impurity band has a normal dispersion.