It is well known that HCCI combustion has low ignition timing controllability and that this characteristic has been a major obstacle for automobile manufacturers to apply this combustion into production engines. In this study, to overcome the drawback, diesel injection was used to control and stabilize the combustion. The experiment results indicate that this combustion concept is effective to stabilize HCCI combustion. At medium load conditions, while attaining extremely low NOX and PM emissions, high thermal efficiencies were achieved. Simulation results showed reasonable agreement with the experiment results, thus visualization of the combustion was realized.