This study explores the use of two conventional ignition improvers, 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP), to enhance the autoignition of the regular gasoline in an homogeneous charge compression ignition (HCCI) engine at naturally aspirated and moderately boosted conditions (up to 180 kPa absolute) with a constant engine speed of 1200 rpm. The results showed that both EHN and DTBP are very effective for reducing the intake temperature (Tin) required for autoignition and for enhancing stability to allow a higher charge-mass fuel/air equivalence ratio (ϕm). On the other hand, the addition of these additives can also make the gasoline too reactive at some conditions, so significant exhaust gas recirculation (EGR) is required at these conditions to maintain the desired combustion phasing. Thus, there is a trade-off between improving stability and reducing the oxygen available for combustion when using ignition improvers to extend the high-load limit. Because previous works have shown that partial fuel stratification (PFS) can be applied with more reactive fuels to reduce the heat release rate to allow higher loads or more advanced combustion timing without knock, the potential of the ignition improvers to allow effective PFS was also explored over the same range of intake pressures. The effect of the additives on NOx emissions was also studied. The results showed that NOx emissions increase with increased EHN concentration but are not affected by DTBP. This work indicates that conventional ignition improvers can effectively enhance the HCCI autoignition reactivity of conventional gasoline at naturally aspirated and modestly boosted operations, offering significant benefits for HCCI engines.