The effect of grain boundary segregation (GBS) on grain boundary diffusion (GBD) is analyzed in frame of the new model. This model supposes the formation of the specific “nuclei” in a grain boundary (GB) solid solution. Their composition is close to that of the nearest phase in grain in equilibrium with solid solution in grain. These GB “nuclei” form after the same solubility as in the bulk is reached. The size of these “nuclei” is close to an atomic size and consequently the new interfaces inside the two-dimensional GB are not formed. As the solute concentration in the bulk increases the solute concentration in GB increases as well, but only due to the increase of the “nuclei” fraction. At the same time the solute concentration in a disordered part of GB solid solution remains constant. The fraction of this part decreases. The retardation effect of GBS on GBD is connected with the ordering transition, the “nuclei” formation. A diffusion coefficient (D) in an ordered part of solid solution is close to the diffusion coefficient in a bulk phase. As a rule, it is less than a diffusion coefficient in a disordered part. The growth of an ordered part of GB solid solution (the fraction of the “nuclei”) leads also to the dependence of D on the solute concentration and to the additional curvature of the diffusion profiles.