Unsaturated brine of a known concentration of NaCl was added into a solution from a NaCl fluidized bed crystallizer to make unsaturated state and the change of fines in the solution was studied against operational conditions of unsaturation and agitation time. The population of fines counted after they became visible by standing of the solution in a supersaturated states for 24 hours is correlated with the product between unsaturation and agitation period, and the dissolution coefficient is obtained. A dimensionless design chart of product crystal size, production rate, suspension density of crystals in a crystallizer, average nucleation rate and average growth rate is proposed, and an industrial application for manufacturing products having desired crystal size by the partial dissolution of fines is discussed.