Diesel/natural gas dual fuel (DNDF) and diesel/methanol dual fuel (DMDF) both have very good potential for use in alternative energy technologies for internal combustion engine. In this work, a comparative study was done to better understand the ignition characteristics of dual fuel combustion. First, their apparent ignition characteristics were studied experimentally on a constant volume chamber and apparent ignition delay correlations of DNDF and DMDF were built; second, chemical kinetic analysis was used to explain the phenomenon observed; lastly, an improved KIVA-3 V code was applied to study the importance of the chemical interaction between diesel and methane or methanol on diesel ignition timing in an engine. Experimental results showed that both methanol and methane addition retarded the ignition of diesel and methane retarded less, which was due to the dehydrogenation of methanol and methane, reaction CH3 + HO2 = CH3O + OH and the formation of H2O2. CFD numerical results showed that, in the DMDF engine, the chemical interaction by methanol addition was the most important factor for the retarding of diesel ignition timing compared to other factors, such as the reduction of the amount of diesel, lower oxygen concentration, and the higher specific heat in the intake charge; in the DNDF engine, the effects of the chemical interaction and the reduction of the amount of diesel were comparable.