A wavelength demultiplexer based on photonic crystal waveguide (PCW) couplers is presented. A numerical approach based on the two-dimensional finite-difference time domain is employed to analyse, design and optimise the wavelength demultiplexer based on photonic crystal structure. The performance of the device is investigated in terms of optical efficiency, device length and losses. Significant improvement on the power efficiency and the wavelength directionality has been achieved by introducing a single defect on the crossing point between two PCWs.