In this study, the problem of GCI method at low intake air temperature was solved by using gasoline-biodiesel blended fuel. Autoignition characteristics of gasoline-biodiesel blended fuel was analyzed using a rapid compression expansion machine. Gasoline-biodiesel blended fuel was mixed 20% of biodiesel with gasoline volume and spontaneous ignition characteristics were analyzed according to fuel injection timing and compression ratio. Firstly, this study investigated spontaneous ignition characteristics of gasoline-biodiesel blended fuel according to the injection timing. A compression ratio was fixed at 12, and the injection timing was delayed from −49° CA ATDC to 0° CA ATDC. The ignition delay time of the mixed fuel was decreased and the IMEP was increased as the injection timing was delayed until −10° CA ATDC. However, the ignition delay time and IMEP subtly changed when the injecting was near TDC. To investigate the auto-ignition characteristics of the gasoline-biodiesel blended fuel according to the compression ratio, the experiment tested at 10, 12, 14, and 16. At that time, the injection timing was set to −33° CA ATDC, −20° CA ATDC, −10° CA ATDC, and −0° CA ATDC. The ignition delay tended to be shorter as the injection timing was delayed at all compression ratios but IMEP calculated at CR10, 16 is good at only specific injection periods. At the compression ratios 12 and 14, the most extensive range of IMEP results were obtained.