In order to investigate the effects of a DC electric field and its polarity on the knock intensity in a spark-ignition engine, an experimental study was carried out with a rapid compression machine. To get a good understanding of the effect of an electric field on knocking combustion, the high-speed direct photographs were taken. The ionization current measurements were also carried out using the electrode as an ionization probeThe major findings of present investigation of the effects of DC electric fields on the knocking combustion process in a spark-ignition engine could be summarized as follows:It was clearly indicated that the knock intensity decreases with the increase of the electric field regardless its polarity. The knock intensity was strongly dependent upon the burned mass fraction at the onset of the end-gas autoignition, and decreased as the burned mass fraction increased. It was found that the electric field causes the increase of the flame propagation speed, but even in the case of the longest electrode, L-electrode, the increase in the burned mass fraction is not so much. It was also observed that the reduction rate of the knock intensity depends upon the ionization currents.