In the reaction crystallization of sodium fluoride in a semi-batch reactor, the effect of the crystallization conditions, such as the reactant concentration and agitation speed on the crystal configuration, are characterized by the appearance of irregular surface crystals on the mother crystals. As the reactant concentration and agitator speed increased, the number of surface crystals on the mother crystals increased; on the other hand the mean crystal size, which included the mother and surface crystals, decreased. These experimental results can be explained by secondary nucleation on the mother crystals and agglomeration of free nuclei and small crystals. A higher interfacial supersaturation contributes to secondary nucleation and a higher collision frequency contributes to agglomeration. These conditions arise with an increase in the reactant concentration and agitator speed, and result in more surface crystals and smaller mean crystal sizes.