In order to meet increasingly stringent fuel consumption and emission regulations, more attentions are paid to engine efficiency. A large quantity of energy-saving technologies have been applied in automotive field especially in gasoline engines, such as turbo-downsizing-direct injection, cooled EGR, long stroke, high tumble and so on. However, the efficiency of gasoline engine is still lower than advanced DI diesel engine. Thus, further improvement in thermal efficiency is essential. It is known that lean burn and ultra-high compression are two root methods to increase efficiency. In this paper, a rapid compression machine is employed to study the combustion process of lean iso-octane mixture at ultra-high compression ratios. The ignition of the mixture is triggered by a spark plug at the timing right after the end of compression, then the flame propagation and end-gas auto-ignition is recorded using high-speed photography. The effects of equivalent ratio, compression ratio and dilution ratio on the combustion process are investigated. The results indicate that controlled end-gas auto-ignition in the lean mixture are viable under high pressure and medium temperature, which might provide a promising directions for higher efficiency of engine.