A new method of determining the knock detection level, which is very important for KCS, has been developed on the basis of the following statistics. The logarithmic transformed value of the knock sensor signal peak level showed its normal distribution on non-knock conditions, which distribution was, however, diverted to its larger side under knock conditions. The difference of distributions was clarified on the probability paper. The former formed a straight line, while the latter did an broken line with a break point, indicating a frequency of knock occurrence. This break point was employed to obtain the KCS with its optimal knock detection level.