The correlation between the autoignition characteristics of n-butane/di-methyl ether/air mixtures and the knock intensity in a rapid compression and expansion machine was studied. The propagation velocity was obtained from the images of propagating flames at the moment of autoignition by a high-speed camera. The dimensionless parameters, ε and ξ, were used in the investigation, which are based on the theories of Zeldovich and Bradley. To obtain these dimensionless parameters, the numerical calculation using CHEMKIN-PRO with AramcoMech1.3 was carried out. The smaller gradient of autoignition delay time for n-butane with the higher amounts of di-methyl ether addition caused the higher flame propagation velocity and the resultant higher knock intensity, while significant differences of excitation times for autoignition heat release could not be seen. As a result, the gradient of autoignition delay time was concluded to have a strong influence on the knock intensities.