Solid state diffusion is of great theoretical and practical importance. The tracer-technique can be applied to help solve many theoretical and practical problems. In this paper the thin layer method was used to study the diffusion of Cu-64 in different carbon-steels. Experiments were performed at temperatures of 1173, 1193, 1213 and 1233 k in order to determine the diffusion coefficients of Cu-64 in steels containing 0.2, 0.4, 0.6 and 0.8 wt % of carbon. From the results the effect of carbon content on the diffusion of Cu in steel was evaluated. Also the activation energies for the diffusion of Cu in the different specimens were calculated.