The effect of the premixed ratio and supply condition of premixed fuel on the combustion and emission characteristics of a partial homogeneous charge compression ignition (HCCI) engine has been investigated experimentally and numerically. The fuel, which is supplied by an additional port fuel injector in the intake manifold, is premixed with air. The homogeneous charge is compressed and ignited by directly injected diesel fuel in the combustion chamber. In this work, the experimental investigation is made by varying the supply conditions of the premixed fuel, such as the injection pressure and the intake temperature. Results show that a partial HCCI engine with a premixed gasoline fuel shows single-stage ignition and the increase in premixed ratio of gasoline fuel results in the advance of ignition timing and the remarkable reduction of nitrogen oxide emission of the engine. Also, heating the intake air is useful for reducing the HC and CO emissions, which are regarded as one of the problems in the partial HCCI engine.