Surface passivation of AlGaN/GaN heterojunction structure was examined through the thermal oxidation of evaporated Al. The Al-oxide passivation increased channel conductance of two dimensional electron gas (2DEG) on the AlGaN/GaN interface. The sheet resistance of 463 ohm/□ for 2DEG channel before Al2O3 passivation was decreased to 417 ohm/□ after passivation. The oxidation of Al induces tensile stress to the AlGaN/GaN structure and the stress seemed to enhance the sheet carrier density of the 2DEG channel. In addition, the Al2O3 films formed by thermal oxidation of Al suppressed thermal deterioration by the high temperature annealing.