Simple optical demultiplexer is realised in this paper using photonic crystal fiber (PCF), where photonic crystal fiber acts as filter, that controls the permission or forbidden of input signal through it, which is the basic principle of operation of PCF based demux. Plane wave expansion method is employed in the present research to compute the electric field at out put end to envisage the above said operation at three communication windows. Finally, the present work divulges that aside nature and properties of material of PCF, the configuration of fiber including lattice spacing and diameter of air holes plays vital role to understand the filtering as well as DEMUX application.