The Mach-Zehnder Interferometer is one of the fundamental components in a photonic circuit. It has a variety of functions, such as wavelength filtering, optical switching, and optical sensing. Gallium Nitride (GaN) semiconductor-based devices have been a source of interest for photonic device researchers, owing to their ability to operate at high temperatures and high power levels. In this work, we have investigated the effect of wavelength variation on the relative power in the Mach- Zehnder Interferometer structure, based on two directional couplers using GaN on Sapphire as a preliminary study to design the wavelength selections. The structure was optimised using OptiBPM with the beam propagation method. We optimised the structure by varying length of the waveguide and the gap between the top and bottom waveguide. Based on the results of the numerical experiment, we found the best length and gap value of the Mach-Zehnder Interferometer to be 14500 μm and 36μm. It also shown that optical field propagation is uniform and the relative power reaches 0.93 at a 1.55μm wavelength. The simulation was conducted using a wavelength range from 1.50 up to 1.60 μm with increments of 0.01 μm to obtain the performance of structure at the C-band range. By varying wavelength from 1.50 up to 1.60 μm, the result indicate that the optical field propagation is uniform and the relative power almost stable at 0.9.