A special phenomenon, anisotropic diffusion is observed during plasma-assisted nitriding of a nickel-based alloy, Inconel 690. The diffusion rate is found to be higher in grains with an orientation close to <100> than in those grains with an orientation close to <111>. A model is established basing on the anisotropic strain and stress during nitriding so as to explain this phenomenon. It is shown that, the anisotropic stress in this f.c.c. alloy causes the different activation energy of N on various grain orientations, and finally result in the anisotropic diffusion rate of N in the alloy. Based on this model, a good agreement between calculated results and experimental results is obtained for temperatures below 400 °C.