Abstract: Performances of different combined cycle engine concepts, including Turbine/Ramjet/Dual-mode Scramjet (TE/RJ/DMSJ), Turbine/Ejector Ramjet/Dual-mode Scramjet(TE/ERJ/DMSJ), Pre-cooled Turbine/Dual-mode Scramjet (PCTE/DMSJ) and Air-turbo-rocket/Dual-mode Scramjet (ATR/DMSJ), are studied. The altitude-velocity characteristics of the combined cycle engines are obtained. Based on the same mission and aerodynamic characteristics of a hypersonic vehicle capable of Ma 6.5 cruise, the range, cruise distance and acceleration time are calculated of the vehicle by using the different combined cycle engines. The results indicate that the TE/RJ/DMSJ has the highest specific impulse among the four kinds of engines. The vehicle has the longest range and acceleration time by using TE/RJ/DMSJ under the same thrust loading and wing loading and it has the shortest range and acceleration time by using ATR/DMSJ.