Low NOx combustion is possible by PREDIC (PREmixed lean DIesel Combustion) in which fuel is injected at a very early stage of the compression stroke and the combustion starts at near the top dead center by self-ignition. To simplify the phenomenon of the PREDIC process, the test engine was operated with gaseous fuels added to intake air to realize combustion of a perfectly homogeneous mixture. The rich limit was observed around λ=2.0∼2.4. This limit was determined by considering the increase in NOx, and the steep pressure rise. During high load operations is not only the ignition timing but also the combustion rate should be controlled. By comparing the homogeneous charge and direct injection case, the mixture heterogeneity could be found to have an influence on the ignition timing and combustion rate, the engine speed and injection timing also had an influenced on these.Since the PREDIC is controlled mainly by chemical reactions, the measurements of the concentration of major combustion species can be very useful to clarify the nature of the reaction process. So using a high-speed gas sampling method, some critical chemical factors controlling the ignition timing was investigated by adding various inert gases from intake pipe. The low temperature reaction, partial oxidation and thermal decomposition process were investigated. With an inert gas addition, the build-up time of aldehydes became longer, the thermal decomposition species such as propylene and ethylene were increased more than without any diluent addition. The oxidation process proceeded in a moderate fashion.