The effect of Research Octane Number (RON) on the ignition delay from Homogeneous Charge Compression Ignition (HCCI) has been experimentally investigated By varying the RON, it was possible to ascertain whether or not RON is the main resource of Ignition delay. Additionally, the information on Equivalence ratio for varying RON was obtained. The tests were performed on a RCM (Rapid Compression Machine) fueled with n-Heptane and iso-Octane. The result showed that increasing RON (0,70,80,90,100), the ignition delay of low temperature reaction (tL) and high temperature reaction (tH) is longer. And the temperature of reaction increases by about 30K. A similar comparison of the effect of Equivalence ration on ignition delay at RON(70,80,90), tL and tH is longer.