In a directional coupler based on photonic crystal waveguides (PCWs), the coupling coefficient can increase by reducing the refractive indexes of the rods between the two coupled PCWs. Accordingly, a shorter beating length and ultracompact device can be achieved. The distribution of odd mode intensity is nearly unaffected, but the distribution of the even mode intensity shifts to higher frequencies when the indexes are reduced. Furthermore, the beating length slope increases for a fixed frequency range, and this technique allows smaller channel spacing and more signal channels for a given total device length. The finite-difference time-domain method is used to validate the behavior that the beating length can be shortened by reducing the refractive index of the modified rods.