In order to be able to use homogeneous charged compression ignition engines industrially, it is necessary to solve the problems of it such as misfiring at low loads and knocking at high loads. In this study, the effects of the intake air temperature and air/fuel ratio on combustion characteristics of a port type single cylinder HCCI engine were examined experimentally by using RON20 fuel. Experiments were carried out at intake temperatures of 40 degrees C, 60 degrees C, 80 degrees C and 100 degrees C and different air excess coefficients using RON20 test fuel to determine the operating range of the HCCI engine. It was seen that the operating range of the HCCI engine was extended and HCCI combustion was obtained with more lean mixtures. The results showed that the cylinder pressure and hear release rate were increased with the increase of intake air temperature. Furthermore, the increase of the intake air temperature caused the combustion duration to shorten and the location of the maximum cylinder pressure was advanced. The occurrence of the maximum cylinder pressure prior to top death center caused a decrease in thermal efficiency. Operating range expanded by increasing intake air temperature, however, due to decrease in the density of air taken into the cylinder, the amount of the oxygen had also decreased and it was lead to decrease in indicated mean effective pressure. Increasing the intake air inlet temperature increases the velocity of the molecules and improves the combustion. So that HC and CO emissions tend to decrease. Increasing the intake air temperature causes the temperature of the combustion of increase. Therefore, NOx emissions also increase.