Partially premixed combustion (PPC) is a low-temperature combustion concept that could deliver higher engine efficiency, as well as lower emissions. Gasoline-like fuel compression ignition (GCI) is beneficial for air/fuel mixing process under PPC mode because of the superior auto-ignition resistance to prolong ignition delay time. In current experiments, three surrogate fuels with same research octane number (RON77) but different octane sensitivities (OS), PRF77 (S = 0), TPRF77-a (S = 3) and TPRF77-b (S = 5), are tested in a full-transparent single cylinder AVL optical compression ignition (CI) engine at low load conditions. Aiming at investigating the fuel octane sensitivity effect on engine combustion behavior as well as emissions under GCI-PPC mode, engine parameters, and emission data during combustion are compared for the test fuels with a change of injection timing. In addition, in order to get a deeper insight into fuel OS effect on GCI-PPC mode, high-speed natural flame luminosity (NFL) imaging techniques are used for visualizing in-cylinder combustion processes. The results show that higher octane sensitivity generally lead to delayed start of combustion, prolonged ignition delay time, retarded combustion phasing of CA50 as well as extended combustion duration. At late injection timing of-15 CAD aTDC, higher IMEP is achieved with higher OS fuel along with lower maximum in-cylinder pressure. A 4% increase of IMEP is achieved by the test fuel with highest OS (TPRF77-b) as compared with zero sensitivity fuel (PRF77). Moreover, the fuel spray vaporization process is affected by fuel octane sensitivity, which introduces some bright tiny spots during the combustion process as well as higher UHC and CO emissions, especially for early injection timing.