Ethanol, one of the most widely used biofuels, has the potential to increase the knock resistance of gasoline and decrease harmful emissions when blended with gasoline. However, due to the characteristics of ethanol, a trade-off relationship between knock tolerance and BSFC exists which is balanced by the blending ratio of gasoline and ethanol. Furthermore, in a spark-ignited engine, it is reported that the blending ratio that maximizes thermal efficiency varies based on the engine operating conditions. Therefore, an injection system that can deliver gasoline and ethanol separately is needed to fully exploit the benefit of ethanol. In this study, PFI injectors and a DI injector are used to deliver ethanol and gasoline, respectively. Using the dual fuel injection system, the compression ratio was increased from 9.5 to 13.3, and the knock mitigation characteristics at the full load condition were examined. Changes in thermal efficiency and emission characteristics were also investigated at full load and part load conditions. From the experiment, it was confirmed that knock was suppressed for both compression ratios when ethanol was added, and an increase in the maximum IMEP was observed consequently. The addition of ethanol also contributed to the decrease of THC and particulate emissions while the NOx emission level did not substantially change.