This paper focuses on a novel combustion mode that is developed from the original concept of homogeneous charge compression ignition (HCCI) combustion. Experiments on iso-octane thermo-atmosphere combustion were carried out on a modified single-cylinder engine. n-Heptane was induced from the intake manifold to promote the ignition of iso-octane spray that was directly injected into the cylinder, through HCCI combustion of n-heptane. The combustion and emission characteristics of this novel combustion strategy were analyzed, and the effects of the iso-octane supply advance angle and n-heptane premixed ratio were investigated. The results show that the influence of the thermal effect and active radical produced by n-heptane HCCI combustion on the iso-octane ignition and emission is varied, depending upon the iso-octane supply advance angle. With the advancing of this fuel supply angle, the ignition of iso-octane is enhanced at first but then suppressed. At an iso-octane supply advance angle of 25 degrees CA BTDC, iso-octane is best ignited and the highest combustion efficiency and indicated thermal efficiency are obtained. Meanwhile, the level of CO in the exhaust gas is the lowest, with the penalty of the worst NOx emission.