GaN-based structures have attracted many researchers in developing photonic devices. These semiconductor structures can operate at high temperatures and high-power levels due to their mechanical hardness. So far, optical splitters design based on Y-junction splitters are widely used on the various material structure. The problem of these structures is radiation loss at the junction area. In this paper, we study the effect of various waveguide parameters on GaN-based S-bend Y-junction optical power divider. The design consists of three sections: linear rectangular rib waveguide, parabolic taper and two S-bend sine branches with the total length of 1000μm. We use optical beam propagation methods (BPM) to obtain the characteristic of the guided wave to calculate the field propagation through the structure as a function of waveguide parameters, such as branching angle and input wavelength. All simulations are carried out using the OptiBPM software. From the simulation results, it shows that to maintain 90% transmitted power, the branching angle for the design should be at the range of 0.5-1.5°. It is also demonstrated that the transmitted power almost stable through the wavelength range from 1.5 up to 1.6 μm with an average of power loss at about 0.35 dB. The simulation results presented can be applied for future GaN Y-junction based waveguide photonic devices design.