It is shown that treatment of GaN epilayers by a low energy low dose focused ion beam with subsequent photoelectrochemical etching represent an efficient tool for GaN nanostructuring. Direct “writing” of surface negative charge trapped by radiation defects allows one to fabricate thin GaN walls with the thickness as low as 100 nm using focused ion beam treatment. The obtained results show that the undercut etching inherent to GaN etching through windows defined by surface charge lithography depends on the depletion length in doped GaN material and does not occur in the structures below critical size of 200 nm in our case.