Abstract — High transmission losses are the key problem that limits the performances of visible light communication (VLC) system that work on wavelength division multiplexing (WDM) technology. In order to overcome this problem, we propose a novel design for a 1×4 optical demultiplexer based on the multimode interference (MMI) in slot-waveguide structures that operates at 547 nm, 559 nm, 566 nm and 584 nm. Gallium-nitride (GaN) and silicon-oxide (SiO2) were found to be excellent materials for the slot waveguide structures. Simulation results show that the proposed device can transmit 4-channels that work in the visible light range with a low transmission loss of 0.983-1.423 dB, crosstalk of 13.8-18.3 dB and bandwidth of 1.8-3.2 nm. Thus, this device can be very useful in visible light networking system that works on WDM technology.