Attempts were made to measure knocking phenomenon by an optical method, which is free from influences of mechanical noises and is allowing an easy installation to an engine. Using a newly developed high durability optical probe, the light intensity of hydroxyl radical component, which is diffracted from the emitted light from combustion, was measured. The intensity of this emission component was measured at each crank angle and the maximum intensity in a cycle was identified. After that, the angular range in which the measured intensity exceeded 85% of this maximum intensity was defined as “CA85”. When a knocking was purposely induced by changing the conditions of the engine operation, there appeared the engine cycles that included CA85 less than a crank angle of 4 degrees. The frequency of occurrence of CA85 equal to or less than 4 degrees within a predetermined number of engine cycles, which can be interpreted as a knocking occurrence ratio, was denoted as “CA85-4”. In order to evaluate the relationship between knocking and its associated damage, “KDI” was defined as a parameter that was calculated by multiplication among CA85-4, which represents frequency of knocking occurrence, and the volumetric efficiency, which represents the amount of heat supplied per unit cycle, and engine speed, which represents the number of combustions within a unit time. Correlation was found between the KDI and the operation time period until the engine sustained the damage. The results showed that the correlative relationship between KDI and the operating time in which the engine can withstand until experiencing damage.