Abstract: The widths of metastable zone for aqueous solution of potassium chloride were determined in a constant cooling rate set. Under the seeded and unseeded conditions, two unparallel log b-log △θmax lines were obtained. The slope of the line of seeded was less than that of the unseeded. The width of metastable zone is dependent not only on the rate of cooling, but also on the retention time. Based on three groups of experimental data of different conditions, an empirical correlation of metastable zone width (△θmax), cooling rate( b ), and retention time(τ) has been found: b = 0.443 △θmax3.167(τ+1)0.904 The rate of agitation of the apparatus was determined by Laser Doppler Velocimeter (LDV). A prelimenary study of the effect of the rate of agitation on metastable zone was made.