Batch cooling crystallization of potassium alum in aqueous solution was conducted over a wide range of seed loading with various average seed sizes under the natural cooling condition. A solution of a low saturation temperature was used. In this dilute solution the supersaturation during crystallization did not become high so that agglomeration of the original seeds was avoided even for small seeds. The critical seed loading ratio C-s*, the critical value of the ratio of seed mass to the theoretical crystal yield, above which loading seed crystals were grown with no secondary nucleation, was determined. It was correlated as a second order equation of mean mass size L-s [mum], as C-s* = 2.17 x 10(-6) L-s(2). The value of C-s* was confirmed, by comparing with previous data published by the present authors, not to change significantly with crystallizer volume, stirrer speed, the saturation temperature of the solution (i.e., the solution concentration) and the cooling mode. This correlation is shown to be conveniently used to determine the seed amount satisfying an optimal seeding condition of C-s greater than or equal to C-s* for a given seed size. The mean mass size of product crystals and the volume of a crystallizer required for a desired production is easily determined for this optimal seed loading condition by the aid of a simple mass balance.