Nitridation of Corning 7101 glass substrate and the following GaN deposition were carried out in a self-developed electron cyclotron resonance plasma enhanced metalorganic chemical vapor deposition (ECR-PEMOCVD) system equipped with in-situ reflection high-energy electron diffraction (RHEED) monitoring. RHEED pattern and X-Ray diffraction (XRD) spectrum showed that the nitridation can effectively improve the C-axis orientation of as-prepared GaN film. Atomic force microscope (AFM) analysis indicated that the average grain size increased significantly with 5 min of nitriding, but degraded as nitriding time increased. The optimum nitriding time was achieved as 5 min. The effect of nitridation on the GaN film deposition and its formation mechanism were discussed.