A comprehensive understanding of the effects of free radicals on the ignition properties of practical fuel is critical for the performance of hypersonic vehicles. In this study, the free radical effects of H, CH3, and C2H5 on the ignition delay times of methane/air mixtures at dosages of 10−7–10−2 (mole fraction) were systemically analyzed via kinetic analysis with two detailed mechanisms (i.e., AramcoMech 2.0 and USC MECH II). Results showed that the addition of free radicals mainly promoted the ignition process at low temperature (800 K), while the addition of molecule H2 mainly improved the ignition process at high temperature (1250 K). When the additional fractions of free radicals were less than 10−3, promoting effects of the three kinds of free radicals were the same. Differences appeared only when the contents were higher than 10−2. Further kinetic analyses indicated that the addition of free radicals could change the critical reactions during the ignition process of methane/air mixtures. The ignition delay times were reduced by increasing the generation rate of OH radical and heat release rate with the addition of free radicals, while the overall output of OH and flame temperature were not affected by the free radicals.