In this study, the effect of the low octane number fuel on HCCI engine performance was experimentally investigated using a slightly modified commercial four-cylinder gasoline engine. To operate the engine in HCCI strategy with wide operational range, the blowdwon supercharging (BDSC) system proposed by the authors was applied in the test engine. Research octane number (RON) of test fuels was varied from 90 to 78.5 as an experimental parameter. Experimental results showed that in the range of the present study, HCCI operational range, brake thermal efficiency and exhaust emissions during HCCI operation were little affected by the RON of the test fuels. In contrast, during SI operation, thermal efficiency was deteriorated with lower RON fuel because of knocking. In terms of “Well-to-Wheel” energy consumption, it is favorable to use gasoline without any additional treatment for increasing fuel octane number, because additional energy is necessary for increasing octane number of commercial gasoline. It is expected that the “Well-to-Wheel” CO2 emission can be reduced by application of high efficiency HCCI combustion into production engine operation with low octane number fuel.