Under heavy load condition, single fuel operation with diesel fuel was studied experimentally for the homogeneous charge diesel combustion (HCDC) method. HCDC engine, in which pre-mixture was formed by fuel injected into an intake manifold and mixed with air beforehand then ignited by small amount of fuel directly injected into a cylinder, can reduce NOx and smoke simultaneously from the diesel engine. In HCDC the higher the premixed fuel ratio was, the lower the emissions were. Accordingly, it was indicated that homogeneous pre-mixture contributed to improvement of exhaust emissions. However, a diesel knocking due to uncontrolled self-ignition may occur under high premixed fuel ratio conditions in the case of operating heavy loads. Thus, the maximum amount of premixed fuel was restricted by these knocking limits. Full load operation was possible without increasing emissions by using moderate compression ignition of premixed fuel and combustion of direct injection fuel at the same time with controlling the rate of premixed and direct injection fuel. Because it was indicated that most of emission originated from direct injected fuel, optimization of intake charge pressure and start of injection timing were tried. Consequently, both NOx and smoke mass could be reduced to half. Furthermore a oxygenated fuel additive which can improve especially smoke was attempted for additive of premixed fuel.