Investigations of Homogeneous Charge Compression Ignition (HCCI) combustion have been actively conducted as a new combustion technology to substantially and simultaneously reduce NOx and soot to comply with the future stringent exhaust emission regulations. In the past, a method of injecting fuel at the initial stage of the compression stroke has been proposed, but it is known that fuel adheres to the cylinder wall, causing a decline in combustion efficiency and oil dilution. The authors have developed Premixed Compression Ignition (PCI) combustion as a technology of solving the above problem as well as simultaneously reducing NOx and soot. In PCI combustion, fuel is injected into a combustion chamber in the vicinity of the top dead center for preventing fuel from adhering to the wall, and pre-mixture, which is formed shortly before ignition, is burnt. By pre-mixing, this combustion reduces the over-rich region of the mixture to reduce soot emissions, and at the same time lowers the combustion temperature by introducing a large amount of EGR to reduce NOx emissions.This paper reports the result of detailed examination of the basic characteristics of PCI combustion using a single-cylinder engine, and that PCI combustion which uses our investigated approaching can achieve substantial and simultaneous reduction of NOx and soot. This paper also studies the possibility of realizing Split-PCI combustion, which uses the two different combustion modes of PCI combustion and diffusion combustion during one cycle, in high-load operation where application of PCI combustion is restricted by diesel knock. As results of this study, this paper reports that it is available to reduce NOx and soot emissions to a large extent by Split-PCI combustion, even in high-load operation.