In this paper a T-branch optical diplexer in two dimensional (2D) photonic crystal (PhC) to select two telecommunication wavelengths 1493.6nm and1553nm is investigated. In our design directional couplers (DC) and resonant cavity (RC) are utilized. A square lattice of silicon (Si) rods in air is used as fundamental structure. The coupling regions consist of three entire rows of decreased Si rods. Plane wave expansion method (PWE) and finite difference time domain (FDTD) method are utilized to analyze and simulate the characteristics of the designed device. The average transmission efficiency of our proposed diplexer is about 99.75%. High quality factor and extremely small crosstalk were achieved. The total size of the suggested design is 272.214 μm2, which is very suitable for nanotechnology based demultiplexing applications.