We performed a feasibility study on the solid-phase diffusion of carbon into GaN using a SiNx/CNx/GaN structure prepared by electron-cyclotron-resonance-assisted chemical vapor deposition. An X-ray photoelectron spectroscopy study on the CNx layer deposited on GaN showed that its energy positions and spectrum features are very close to those of a C-N bond, and the N composition was estimated to be 24 %, indicating a highly C-rich layer. No degradation in the chemical properties of the GaN surface was found after the diffusion process at 1000 ℃. A secondary ion mass spectrometry result clearly showed a diffusion of carbon into GaN. We also observed an increase in resistivity for the C-diffused GaN layer.