An improved semi-batch cooling crystallization of potassium alum (AlK(SO₄)₂·12H₂O) was developed to produce monodisperse crystals under the unseeded condition. The two-stage cooling was a useful method for the prevention of secondary nucleation and the size enlargement of generated fine crystals because supersaturation is successfully controlled by the selection of the optimal feed condition in each cooling stage. In this study, the calculation and validation of semi-empirical feed condition was carried out based on the seeding policy. Unseeded crystallization technique is particularly important in the field of foods and pharmaceuticals in a view of crystal purity. The semi-batch cooling crystallization is a simple and possible method to produce monodisperse crystals under the unseeded condition which contributes to these industrial fields.