Homogeneous compression ignition (HCCI) engines are an alternative combustion model to existing internal combustion engines due to their high thermal efficiency and low exhaust emissions. In this study, the effects of air fuel ratio and octane number on combustion and engine performance characteristics were investigated with port type injection single cylinder HCCI engine. Experiments were carried out at full load conditions, intake temperature of 60 degrees C and 800 rpm engine speed. Also, experiments were carried out with different air excess coefficients. The maximum in-cylinder pressure occured at a later crank angle by increasing the number of octane. Increasing the octane number has caused the increase of the combustion duration. The increase of air fuel ratio decreases the pressure rise rate. Decreases of air fuel ratio has increased the amount of injected energy to the cylinder. So that, the highest in-cylinder pressures are obtained under these conditions. The highest indictical thermal efficiency 40.6 %, provided that the air fuel ratio was 2.14 with using RON60 fuel. Decreases of air fuel ratio caused the combustion advance. For this reason net work decreased. Thereby, indicated thermal efficiency decreased.