In this paper we describe a new method for in situ monitoring damage density of GaN substrate surface in inductively coupled plasmas (ICP) containing energetic electrons. Energetic electrons are produced by sheath acceleration of secondary electrons at a negatively biased electrode. A current of a Langmuir probe located in such plasma is used to examine behavior of the energetic electrons. When the plasma contains the high energy electrons, a sample of n-type GaN film exposed to the plasma is observed to emit significant optical fluorescence in the wavelength range of 370-390 nm corresponding to band gap energy of the GaN. The fluorescence intensity of the GaN film increases with the incident electron energy higher than a critical value of similar to 5.8 keV. By the XPS results suggested the NBE intensity probably reflects the accumulation degree of the plasma and DC bias induced damage. These results suggest the cathode luminescence technique will be usable to detect a damage density of GaN substrate surface even in plasma conditions. (C) 2013 Elsevier B.V. All rights reserved.