An optimal design of Photonic crystal waveguide (PCW) directional coupler (DC) with adiabatic coupling is presented in this paper. Planar wave expand (PWE) method and FDTD method are used to analyze and optimize PCW-DC. It is shown that use this PCW-DC can achieve efficient transmission ratio in cross state, bar state and power division state at different coupling length. The simulation results show that the coupling ratio can be higher than 90% in some selective wavelength in these states. With the help of adiabatic coupling and bent PCW, the coupling coefficient of two parallel SMFs whose distance from each other at standard interval of 125μm can be higher than 90%. This also suggests that this device can realize a very efficient add/drop filter device which can be used in WDM system.