We demonstrated the fabrication of GaN nanowire (NW) by selective etching of the GaN micro truncated-pyramid (mu-TP) in KOH solution. The GaN mu-TP, which consists of a (0 0 0 1) Ga-polar top surface and six {1 (1) over bar 0 1} N-polar sidewalls, were grown on the patterned AlN/Si template in metal organic chemical vapor deposition (MOCVD) system. KOH solution can selectively etch the N-polar sidewalls while leave the Ga-polar top surface intact. Hexagonal-prism-structured GaN NW with (0 0 0 1) top surface and {1 (1) over bar 0 0} sidewalls was obtained after adequate chemical etching. It was found that the three-dimensional geometry of the GaN NW is determined by the diameter of the (0 0 0 1) top surface and the height of the GaN mu-TP. And the chemical etching mechanism of GaN mu-TP towards GaN NW in hydroxide solution was explained. (C) 2015 Elsevier B.V. All rights reserved.