The detailed combustion mechanism can be coupled with the computational fluid dynamics (CFD) software to simulate the combustion process in a practical engine. However, the computing time may be unfeasibly long. To improve the efficiency of the simulations, a reduced mechanism is preferable. However, the combustion characteristics and prediction accuracy will be influenced by the reduction of the combustion mechanism. In this work, the effects of the reduction of the detailed n-heptane mechanism on the prediction accuracy and efficiency were investigated theoretically. The reduction was based on the directed relation graph method without revising the original kinetic parameters. The results indicated that the reduced combustion mechanism at 1/2 size (1/2 mechanism) of the detailed mechanism performed well and the 1/4, mechanism showed some deviation from the experimental data as a result of the removal of some low-temperature reactions. The 1/8 mechanism performed even worse. An ideal combustion mechanism for coupling with CFD simulations should be of the size between that of the 1/2 mechanism and 1/4 mechanism.