In order to meet increasingly stringent fuel consumption and emission regulations, more attentions are paid to improve engine efficiency. A large amount of energy-saving technologies have been applied in automotive field especially in gasoline engines. It is well known that lean burn and ultra-high compression ratio technologies are two basic and important methods to increase efficiency. In this paper, a rapid compression machine was employed to study combustion process of lean iso-octane mixture at ultra-high compression ratios (16 to 19:1). Regardless of flammability of the mixture, spark was triggered at the timing right after the end of compression, then, the flame propagation and/or auto-ignition can be recorded using high-speed photography simultaneously. The effects of equivalence ratio (φ), compression ratio (ε), dilution ratio, and effective temperature (Teff) on the combustion process was investigated.The results indicate that the controlled auto-ignition, induced by flame under high pressure and medium temperature in the lean mixture, can reduce combustion duration substantially and meanwhile depress the excessive pressure rise rate. Under φ = 0.4, there is an appropriate temperature range Teff = 840-870 K and pressure range Peff = 25-35 bar to maintain the advantages of flame and auto-ignition interactions which might provide a promising way to achieve higher efficiency using ultra-high compression ratios and lean mixture during engine operation.