Abstract: To study the effects of charge compositions on the ignition and combustion process of diesel engine, the flame photos of diesel combustion process with full throttle under different charge compositions were recorded using high-speed camera on the optical engine with observation window on the cylinder-head. The temperature field in cylinder calculated by the three-primary-color method, indicator and heat release rate diagram were applied to analyze the effects of charge compositions on combustion characteristic. The results showed that ignition time，combustion duration，peak value of the cylinder pressure and heat release rate changed with different charge compositions and the changes of charge compositions strongly influenced the maximum temperature and average temperature in cylinder. In addition, intake air mixed with 2% H2 and 10% CO2 improved combustion in reduction of oxygen and depressed the harmful emissions from diesel engine. The study provides a new method for realizing high efficiency and low-pollution emissions in diesel engine.