The optical directional couplers are one of the essential devices in photonic systems. It has many applications such as switches, modulator and de-multiplexer. In this paper, optimisation of the optical directional coupler using Gallium Nitride (GaN) semiconductor on sapphire was reported. The optimisation was conducted by investigating the effect of waveguide parameters on the structure using OptiBPM software. The optimisation result showed that the proposed directional coupler could successfully split light into two output branches. The best length and width of the structure were found to be 1000 μ m and 17.5 μm, respectively, with a coupling gap of 6.0 μm and a width gap of 9.5 μm. It could split light into two output branches with the splitting ratio of 46.76% and 47.75%. The optical field propagation was uniform along with the structure, and the relative power reached 82.25% at a 1.55μm wavelength. The excess loss and power imbalance were 0.84 dB and 0.09 dB, respectively.