The Nox emissions exhausted from diesel engines can be reduced significantly by extremely advancing the injection timing of fuel, because of homogenizing the mixture of fuel vapor and air. In this study, as fundamental research of combustion phenomena in a homogeneous charge compression ignition engine, the ignition delay of light oil spray was investigated under various conditions of cylinder charge. A direct injection diesel engine with a bore of 92mm and a stroke of 96mm was used for the experiment. The degree of homogeneity of mixture was varied by changing fuel injection timing and mixing gaseous fuel (propane and methane) in intake air. Consequently, it was recognized that the ignition delay of light oil injected early was predicted by Livengood-Wu concept for ignition.