The nitrogen molecular dissociation level in 915 MHz ECR plasma was evaluated as a function of the electron temperature by optical emission spectroscopy. The intensity ratio of N line to N-2 line, which provides information on changes in the degree of dissociation of nitrogen molecules, was found to increase monotonously from 0.01 to 0.07 with increasing the electron temperature from 2 to 7 eV. Furthermore, it was confirmed that increasing the electron density and/or the introduction of argon in the discharge significantly enhanced the nitrogen molecular dissociation level. (C) 2003 Elsevier Science B.V. All rights reserved.