In this study, the metastable zone width of potassium tetraborate tetrahydrate was determined for four different temperatures and cooling rates. The induction period of potassium tetraborate tetrahydrate in aqueous solution was examined according to polythermal method by using visual observation. The induction period, which changes inversely proportional to the nucleation rate has been used to determine the interfacial tension between the potassium tetraborate tetrahydrate and aqueous solution. By using interfacial tension, the nucleation parameters such as Gibbs free energy change for the formation of critical nucleus, Delta G*, free energy of formation, Delta G, radius of critical nucleus, r and number of molecules in the critical nucleus, i* has been calculated. The effect of Li+ and Ca2+ impurities on metastable zone width has been studied. The metastable zone width of aqueous solution of potassium tetraborate tetrabydrate decreases with increasing impurity concentrations. The equilibrium saturation concentration change is high in the presence of Ca2+ ions while it is low in the presence of Li+ ions.