We have investigated the transverse mode patte and the optical field confinement factor of gallium nitride (GaN) laser diodes (LDs) theoretically. For the particular LD structure, composed of approximate 4μm thick n-GaN substrate layer, the maximum optical confinement factor was found to be corresponding to the 5th order transverse mode, the so-called lasing mode. Moreover, the value of the maximum confinement factor varies periodically when increasing the n-side GaN layer thickness, which simultaneously changes and increases the oscillation mode order of the GaN LD caused by the effects of mode coupling. The effects of the thickness and the average composition of Al in the AlGaN/GaN superlattice on the optical confinement factor are also presented. Finally, the mode coupling and optimization of the layers in the GaN-based LD are discussed.