A novel wet etching method for AlGaN/GaN heterojunction structures is proposed using thermal oxidation f ollowed by wet etching in KOH solution.It is found that an AlGaN/GaN heterostructure after high temperature oxidation above 700℃could be etched off in a homothermal(70℃) KOH solution while the KOH solution had no etching effects on the region of the AlGaN/GaN heterostructure protected by a SiO_2 layer during the oxidation process.A groove structure with 150 nm step depth on an AlGaN/GaN heterostructure was formed after 8 h thermal oxidation at 900℃followed by 30 min treatment in 70℃KOH solution.As the oxidation time increases,the etching depth approaches saturation and the roughness of the etched surface becomes much better.The physical mechanism of this phenomenon is also discussed. A novel wet etching method for AlGaN / GaN heterojunction structures is proposed using thermal oxidation f ollowed by wet etching in KOH solution. It was found that an AlGaN / GaN heterostructure after high temperature oxidation above 700 ° C could be etched off in a homothermal (70 ° C) KOH solution while the KOH solution had no etching effects on the region of the AlGaN / GaN heterostructure protected by a SiO 2 layer during the oxidation process. A groove structure with 150 nm step depth on an AlGaN / GaN heterostructure was formed after 8 h thermal oxidation at 900 ° C followed by 30 min treatment in 70 ° C KOH solution. As the oxidation time increases, the etching depth approaches saturation and the roughness of the etched surface becomes much better. The physical mechanism of this phenomenon is also discussed.