A GaN-based optical single-mode rib waveguide with a large cross section is systematically designed and analyzed using the finitedifference vector beam propagation method (FD-VBPM). The method is a combination of numerical aperture and FD-VBPM, which is used to determine the single-mode conditions for a GaN/AlxGa1-xN optical rib waveguide. We analyze the optical propagation properties and loss characteristics of the designed waveguides. The analysis illustrates that optical power increases with an increase of the rib width or a decrease of the slab height. Therefore, our analysis results can be helpful in the process of designing a GaN-based rib waveguide for optical communications. © 2006 Society of Photo-Optical Instrumentation Engineers.