It was shown in former experimental studies that the ignition of diesel or n-heptane can be delayed by adding ethanol fuel. However, detailed analysis of the interaction between two fuels in a flame has been lacking. To investigate the underlying mechanism, a constant-volume bomb was used to study the ignition characteristics of diesel in a pure air atmosphere (AA) and an ethanol air mixed atmosphere (EAA). Experimental results show that both the ignition delay and lift-off length of the diesel fuel were prolonged by ethanol fuel. A zero-dimensional model was developed to analyze the effects of ethanol on ignition properties of n-heptane. Simulation results further reveal that ethanol suppresses the low-temperature reaction of n-hepthane, whereas ethanol promotes the high-temperature reaction of n-hepthane. When two fuels are blended, OH is converted into H2O2 in the presence of ethanol, while H2O2 is thus frozen to ignition because of its high activation energy for decomposition. Therefore, the reaction activity before ignition is decreased. Both experiments and simulation show that ethanol plays a dominant role in delaying the ignition of diesel fuel.