The photonic crystal waveguides is quite significant in designing the optical integrated circuits. Half Adder is the important part of any ICs. Here, we have designed an all optical half adder. This adder is designed with T-shaped square lattice photonic crystal. The structure is optimized with the line and point defect to act as Adder. These structure yields output at sum and carry port using phase interference method. Contrast ratio is used to measure the performance. The contrast ratio for sum is 6.50 dB and for carry is 10.47 dB. The footprint of proposed structure is 56 µm². The design is viable to open a way in designing future optical integrated circuits.