Grain boundary self-diffusion in Type-C regime was investigated in nano-crystalline bcc iron at low temperatures between 100 and 250 degrees C using neutron reflectometry in combination with [Fe-nat(7 nm)/Fe-57(3 nm)](10) isotope multilayers. The method allows to determine diffusivities in a stable nanostructure without concurrent grain growth. An activation enthalpy of diffusion of (0.9 +/- 0.3) eV is found, comparable to the value of coarse-grained Fe.