HCCI (Homogeneous Charge Compression Ignition) engine is able to achieve low NOx and particulate emissions as well as high efficiency. However, its operation range is limited by the knocking at high load, which is the consequence of excessively rapid pressure rises. It has been suggested that making thermal or fuel inhomogeneities can be used to solve this problem, since these inhomogeneities have proved to create different auto-ignition timing zones.It has also been suggested that EGR (Exhaust Gas Recirculation) has a potential to reduce pressure rise rate. But according to a past report, it was concluded that under the same fueling ratio and CA50 with different initial temperature and EGR ratio, the maximum PRR is almost constant.The purpose of this study is to investigate the fundamental effects of EGR. First, I considered EGR homogeneous charge case. In this case, the effects of EGR and its components like CO₂, H₂O or N₂ on HCCI combustion process is argued. When diluted with EGR or its components, gas composition changes and the ratio of heat capacity and O₂ concentration become changed. As a consequence, the histories of pressure and temperature, ignition timing and temperature, combustion duration and PRR might be changed. So I checked them with addition of different amounts of EGR and its components under the same fueling ratio, by using RCM (Rapid Compression Machine). I used RCM to avoid influences such as in-cylinder gas temperature and fuel inhomogeneities. Second, I considered EGR inhomogeneous case. In this case, I checked those which are mentioned in homogeneous case with addition of same total amounts of EGR but different amounts of EGR in each zones under the same fueling ratio by using RCM. In these two cases, additional numerical calculation with single and multi-zones model is run to know the potentials of EGR homogeneous and stratified charge, in order to reduce PRR.