Abstract: Combustion characteristics and the variation of in-cylinder key reaction species for DI-diesel engines with such three injection combustion modes as normal injection, split injection and early injection, were investigated by adopting the CFD simulation technique. The results showed that the calculated values of in-cylinder pressure kept in agreement with experimental ones in references, the validity of the model was verified. The in-cylinder pressure with the early injection combustion mode was the highest, the split injection combustion mode took the second place, and the traditional injection combustion mode was the lowest. Among four species (H, N, OH and O) generated inside in-cylinder combustion process, the amount of OH strength was the highest, the N strength and the O strength took the second place, whereas the H strength was the lowest. Hence, it could be concluded that the OH strength was the critical one in the combustion reaction. Besides, the split injection combustion mode was more favorable to improve power, realize stable combustion and reduce emissions compared with the other two modes. The study provides the diesel compression combustion characteristics, and the correlation of reaction species and combustion process under various injection combustion modes.