Unseeded semi-batch cooling crystallization of potassium alum (AIK(SO4)(2)center dot 12H(2)O) was carried out to obtain monodisperse crystal size distribution. A semi-batch cooling method was preferred to batch-wise cooling method for preventing from secondary nucleation because supersaturation was successfully controlled by the feed condition. Further, the semi-batch cooling method provided larger crystals than the batch-wise cooling method, and volume mean size linearly increased with feed rate. In crystallization processes of food and pharmaceuticals, unseeded conditions are occasionally preferred in view of crystal purity. Semi-batch cooling is a simple method to control supersaturation by feed conditions and modify crystal size distribution.