In this paper, a new scheme of all-optical binary half adder is proposed. This scheme realizes a kind of all-optical computation which is based on the effect of spatial property in photonic crystal. The line defect, which is obtained by reducing the radius of host rod, is used to produce the appropriate phase difference between the input signals. The two input signals and the reference signals interfered offer a desired logic function. Because of the device utilizing the linear material, this structure operation is independent of the input power. The proposed adder is demonstrated, numerically, by computing electromagnetic field distribution using finite difference time domain method.