Homogeneous charge compression ignition (HCCI) still faces challenges in high load extension. In this paper, HCCI high load operation range was extended and combustion phasing was controlled by boosting combined with internal EGR and external EGR in a gasoline HCCI engine. Internal EGR was obtained by negative valve overlap (NVO) and used to achieve gasoline HCCI at ambient temperature without intake heating. Combustion phasing was optimized by adjusting the external EGR inducted into the intake system. The experimental results show that both boosting and EGR are effective means for HCCI high load extension but with limitations of peak pressure (P-max), maximum rate of pressure rise (R-max), combustion efficiency (eta(c)), and NOx emissions. Under the acceptable P-max, R-max, eta(c), and NOx levels of a production gasoline engine, the achievable maximum IMEP is in the following order: external EGR, boosting, boosting combined with EGR. With increasing boost pressure, NOx emissions are significantly reduced. But with acceptable P-max heavy boosting leads to an ultralean mixture, followed by uncompleted combustion, which results in CO and HC emissions. At a fixed NVO, there is an appropriate range of the external EGR rate for stable HCCI combustion without knocking or misfire. Therefore, the optimized path was achieved using various internal EGR and external EGR combined to extend HCCI high load operation range. In addition, it can be found that boost pressure, percentage Of CO2 addition, and internal and external EGR rate have shown significant effects on combustion phasing of HCCI.