The optical beam splitter is one of the fundamental components in the optical telecommunication system. In this paper., the design of a 2x2 multimode interference (MMI) beam splitter using gallium nitride-on-silicon structure (GaN/Si) is proposed. The design optimization was conducted by inspecting the optimal MMI waveguide length using the eigenmode expansion method (EME) at the optical telecommunication wavelength of 1550 nm. The optimal design was achieved on the MMI width and length of 15mu m and 212 mu m, respectively. The simulation results showed that the proposed design could successfully split a single input signal from one of the input port into two signals on both output ports with the normalized output power of 0.979., the power imbalance of 0.009 dB., the lowest power output over C-band (1530 nm - 1565 nm) bandwidth of 0.967., and the fabrication tolerance for both MMI waveguide width (lateral) deviation and MMI length (longitudinal) deviation of 0.3mu m and 0.5 mu m, respectively.