For the purpose of elucidating the fouling mechanism, the structures and the properties of the deposit were investigated mainly by using X-ray and electron microscopic techniques.We found that the deposit mainly consists of a graphitic carbon black with highly dense network structure when the insulation resistance of the spark plug was indicating extremely small value. (e.g. below 1MΩ)The cause of the graphitization under low ambient air temperature condition was considered as follows. 1) Catalyzation by transitional metals including iron and/or nickel 2) Sparking energy transfered through the carbon depositFurther carbon depositing processes were observed by several dynamic analyzers such as high speed flame pictures. The results of these analyses show that carbon formation and deposition processes on the spark plug are due to the flame behavior in the “plug pocket” (a space between counter-bore and ceramic insulator), and dropping of the insulation resistance value is caused by many kinds of ions contained in the carbon deposit.