A new concept for diesel combustion was investigated by means of numerical simulation, engine experiment, and combustion observation in order to realize a simultaneous reduction of NOx and particulate emission. This concept (HiMICS: Homogeneous charge intelligent Multiple Injection Combustion System) is based on pre-mixed compression ignition combustion combined with multiple injection. Combustion characteristics of HiMICS concept was investigated by comparing with both a standard single injection and a pilot injection. In HiMICS concept, the pre-mixture is formed by a preliminary injection performed during a period from the early stage of the induction stroke to the middle stage of the compression stroke. Modified KIVA-II code was used to predict engine performances and emissions of each injection method. The simulation results show a capability of considerable improvement in the trade-off relation between NOx emissions and fuel consumption of HiMICS.Engine tests were also carried out to confirm the simulation results using a single cylinder research engine equipped with a common rail injection system. Contrary to the simulation results, the experimental results show that both trade-off relation of HiMICS between NOx emissions and fuel consumption, NOx and smoke emission, becomes worse than those of standard injection or pilot injection in the region of ordinary injection timing. However, on the other hand, HiMICS has an advantage in the fuel consumption and smoke emission in the region where the injection timing is considerably retarded and NOx emission is extremely low. Remaining problems are its high emission level of HC and CO, too early ignition of the pre-mixture, and inadequate homogenization of the pre-mixture.