Potassium alum was crystallized by natural cooling in two well-mixed laboratory-scale (12.2 L) and pilot-scale (600 L) crystallizers. When unimodal product crystals of grown seeds were used, no scaleup effect on product crystal-size distribution (CSD) was observed if enough seeds were loaded over a critical seed concentration. Cooling rate, crystal yield (crystal suspension density), and stirring condition do not seriously affect the product CSD under such a seeding condition. Low supersaturation peak caused by the growth of enough seeds, the short period of nucleation, and the agglomeration of nuclei to the growing seed crystals are considered to be mechanisms working to produce unimodal product crystals of grown seeds. Simple design calculations are demonstrated to determine the amount of seed wit the help of a critical seed concentration, which can be determined easily from laboratory-scale experiments. Crystallizer volume was calculated simply from a mass balance.