One of the most effective means of improving the thermal efficiency and the specific fuel consumption in spark ignition engines is the increase of the compression ratio. However, there is a limit to it because of the generation of knocking combustion due to the rise of temperature and pressure in the unburnt mixture. Also in turbo charged spark ignition engines, the ignition timing cannot be advanced until MBT in order to avoid the knocking phenomena. Generally speaking, it is very difficult to investigate the phenomena in an actual engine, because there are many restriction and the phenomena are too complex and too fast. According-ly, it is advantageous to reveal the phenomena fundamentally, including the autoignition process of the end-gas by using simplified model equipment.Therefore, a rapid compression and expansion machine (RCEM) with a pan-cake combustion chamber was designed and developed for the experiments presented here. The distinctive feature of this machine was the high reproducibility in the compression stroke. Homogeneous and quiescent pre-mixed mixtures of n-pentane with air were charged into the combustion chamber. So-called low speed knocking was initiated in the chamber. The phenomena were observed taking high speed Schlieren photography and the emission intensity of chemical species in the combustion chamber, i.e., OH radical, CH radical, C2 radical and young soot were measured by means of interference filter, in order to get qualitative and temporal data of these radicals. The data are very useful for understanding of knocking phenomena in a actual engine.