This paper describes characteristics of normally‐off mode GaN MISFETs fabricated on free‐standing GaN substrates with high drain current Idss and specific low leakage current between adjacent devices. Nitrogen ion implantation isolation processes were adopted to fabricate isolation regions in self‐aligned GaN MISFETs for the first time. Maximum drain current of 98 mA/mm, maximum trans conductance of 10 mS/mm and threshold voltage of +0.4 V were obtained for the devices. The leakage current between adjacent devices was low, i.e., in the range of 10–6 mA/mm, which is three orders of magnitude lower than those on sapphire substrates. These supreme results indicate a definite availability of normally‐off self‐aligned GaN MISFETs for power switching device applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)