This study examined the octane boosting effects of Î-valerolactone, a fuel derived from lignocellulosic biomass, under Homogeneous Charge Compression Ignition (HCCI) combustion mode. The experiments were performed in a Cooperative Fuel Research (CFR) engine under four sets of conditions defined by the combinations of intake temperatures and rotation speed. Octane boosting effects were rated with FACE (Fuel for Advanced Combustion Engine) J gasoline as a base fuel. Due to the non-miscibility of Î-valerolactone into FACE J, a new approach was proposed in which the octane boosting effect of a mixture comprised up of two-third Î-valerolactone and one-third ethanol was investigated. To evaluate the effect of Î-valerolactone, the octane boosting effect of pure ethanol into FACE J was also investigated such that comparison can be drawn. Further attempts were made to extract the octane boosting effects of pure Î-valerolactone. For convenience, both volumetric and molar approaches were considered to rationalize the experimental results. The results showed that Î-valerolactone is a good octane booster, and that it possesses higher octane enhancement potential than ethanol for a low volume fraction in FACE J. In contrary, opposite trends were observed for the higher volume fractions of Î-valerolactone. Finally, the assessment of the blending octane number for pure Î-valerolactone revealed non-linear octane boosting effects.