Because quench oil deterioration varies depending on the operating conditions, maintaining constant performance is difficult. Various defective phenomena occur in response to changes in the quench oil degradation. In this paper, we will examine how the oil condition, cooling performance, brightness and carbide growth are effected by various defective phenomena such as oil oxidation, thermal decomposition etc. In doing so we would like to introduce important points in managing and setting operating conditions to optimize the performance of quench oils.