A novel directional coupler is designed based on twin core PCF.The optical properties of the device have been demonstrated by finite difference time domain according to the waveguides directional coupling principle.We also study the influence of the structural parameters of the twin core PCF′s cladding.It is found that the coupling length increases with the increasing of d/Λ and decreases with the increasing of wavelength.On the basis of this structure,ultra-compact WDM is designed in details as examples for 0.85/1.55 μm,0.98/1.55 μm and 1.31/1.55 μm.By tailoring other structural parameters,two wavelengths can be separated completely.The results indicate that these properties enable such devices to find great potential applications in wavelength division multiplexing.