A simple model was developed to explain the surface crystals found on the clear mother crystal face. The surface crystals on the mother crystal in the reaction crystallization of sodium were mathematically estimated by using a model based on two-dimensional secondary nucleation and two-step growth mechanisms. The model predicted that the surface crystals found on the mother crystal would be sensitive to the reactant concentration and agitation speed due to their influence on the interfacial supersaturation in the crystal growth. As the bulk supersaturation increased and the molecular transport was facilitated, the number of surface crystals on the mother crystal also increased, because the interfacial supersaturation built up on the mother crystal surface was enhanced. The prediction of the number of the surface crystals on the mother crystal appeared to be consistently comparable with experimental results.