Self-diffusion measurements of iron in nano-crystalline FeN alloy thin film have been performed using secondary ion mass spectroscopy (SIMS) depth profiling technique. Diffusion broadening of a thin marker layer of (FeN)-Fe-57, sandwiched between thick natural FeN layers has been monitored. Structural relaxation effect of the diffusion coefficient has been observed in the grain-boundary region of the alloy. In the relaxed state, the values of the pre-exponential factor Do and the activation energy q are, D-0 = 2x10(-17) m(2)s(-1); q = 0.5 +/- 0.1 eV. In comparison with an amorphous alloy of FeN, with the similar stoichiometric composition, the pre-exponential factor and the activation energy in the present case of nano-crystalline alloy are significantly lower.