The self-diffusion of C in non-stoichiometric UC has been measured, using radioactive tracer and sectioning technique. The diffusion equations finally derived as function of temperature are: The activation energy for migration of C in UC is considered to be about 54kcal/mol. From the dependence of the diffusion coefficient and the activation energy on C content in UC, it was seen that interstitial C atoms significantly influence the C diffusion in UC, and from this the C diffusion mechanism is judged to be either an interstitial or else an interstitialcy mechanism.