In this paper, we studied on a leakage current of a pn diode on a GaN substrate isolated by a mesa structure at a reverse bias. The leakage current flowed at a surface of the mesa sidewall across the pn junction. Results of scanning capacitance microscopy and scanning spread resistance microscopy indicated that the surface of the mesa sidewall in the p type region changed to a depleted or a n--layer. Results of auger electron spectroscopy indicated that a concentration of nitrogen decreased near the surface of the mesa sidewall. From these results, we estimated the leakage current was caused by a change of the p type region at the surface of the mesa sidewall to the depleted or n--layer by inductively coupled plasma dry etching, which was used to form the mesa structure. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)