We systematically characterized gallium nitride (GaN) metal-oxide semiconductor interfaces formed on trench sidewalls, paying particular attention to eliminating the parasitic capacitance caused by the trench bottom. The flatband voltage (V-fb) was found to be clearly higher on the m-face trench than on the a-face trench, and an interface state density of similar to 1 x 10(11) cm(-2 )eV(-1) near the conduction band edge was achieved, irrespective of the trench direction. For trench sidewalls treated with a tetramethyl ammonium hydroxide (TMAH) solution, variation in V-fb among the trench direction was suppressed, reflecting that microscopic m-faces were generated after the TMAH treatment, irrespective of the direction of the trench. Therefore, the origin of the V-fb variation was attributed to the GaN crystal face dependence of fixed charge density at SiO2/GaN interfaces.