Mixture formation and the ignition process in 4 cycle 4 cylinder spark ignition engines were investigated, using an optical combustion sensor that combines fiber optics with a conventional spark plug. The sensor consists of a 1-mm diameter quartz glass optical fiber cable inserted through the center of a spark plug. The tip of the fiber is machined into a convex shape to provide a 120-degree view of the combustion chamber interior.Light emitted by the spark discharge between spark electrodes and the combustion flames in the cylinder is transmitted by the optical cable to an opto-electric transducer. As a result, the ignition and combustion process which depends on the mixture formation can be easily monitored without installing transparent pistons and cylinders. This sensor can give more accurate information on mixture formation in the cylinders. It can monitor cycle-by-cycle, the air-fuel ratio of the mixture near the spark plug by detecting the light intensity of CH and C2 radicals appearing after the spark discharge. Also, it can provide information on fuel film or droplets in the cylinder which cause a diffusion flame, by detecting the light intensity at long wavelengths. This sensor can directly detect cycle-by-cycle, the intensity of a spark discharge by monitoring light emitted by the spark discharge, which precedes combustion flames. The combustion process can be throughly monitored from spark discharge to the end of combustion.