To investigate the effect of premixed fuel on the combustion and exhaust emission characteristics in a partial homogeneous charge compression ignition (HCCI) engine, an experimental study was performed. The premixed fuels are supplied by the additional fuel injection system in the intake manifold of a direct-injection diesel engine and the injection nozzle for directly injected fuel is equipped in the center of the combustion chamber. The results show that. in the case of gasoline as a premixed fuel, single-stage ignition is found. but premixing the diesel fuel is accompanied by a cool flame prior to the combustion of the directly injected diesel fuel. NOx, emissions in the partial HCCI engine with premixed gasoline fuel decrease linearly with increase in premixed ratio. But, for diesel premixed fuel, NOx emission increases at high intake temperature and high premixed ratio conditions.