The nucleation of interstitial loops, in the presence of a defect flow toward the surface, is investigated using a rate theory based model. A di-interstitial is assumed to be the nucleus of a dislocation loop, and both homogeneous and heterogeneous nucleations are considered. One-dimensional diffusion of point defects toward the surface is included in the formulation. Two stages appear in the loop nucleation due to the difference in mobility of interstitials and vacancies. In the first stage, loop nucleation occurs under the condition in which only interstitials migrate toward the surface and are lost. The loop concentration initially increases linearly with time, and then nucleation slows down. The second stage begins when vacancies migrate to the surface, and the loop concentration increases, again linearly with time. The defect-production rate dependence of loop concentration, when compared at a constant time, varies from square to linear as irradiation proceeds. The activation energy of loop nucleation is equal to the migration energy of the interstitial in the first stage, and is equal to the migration energy difference between the interstitial and the vacancy in the second stage. Because the increase of loop nucleation in the second stage occurs earlier near the surface, the depth of the layer devoid of dislocation loops decreases as irradiation is continued.