Abstract: Gas-phase auto-ignition delay times of methane/'air' (21% O2/79% Ar) mixtures were measured behind reflected shock waves, using a kinetic shock tube. Experiments were performed at fixed pressure of 1.8 MPa and at equivalence ratios of 0.5 and 1.0, over the temperature range of 800 - 1000 K. Overall, the effect of equivalence ratio on ignition delay time is negligible at entire temperatures measured in this study. Difference from traditional ignition regime of diluted methane mixtures at high temperatures, the undiluted methane/air mixtures presents a four-stage ignition process at low temperatures, namely deflagration delay, deflagration, transition of deflagration to detonation and detonation. Four popular kinetic mechanism, UBC Mech 2.1, GRI Mech 3.0, Aramco Mech 2.0 and USC Mech 2.0, were used to simulate the new data. Only UBC Mech 2.1 showed satisfactory predictions in the reactivity of undiluted methane mixtures, it was thus adopted to implement sensitivity analysis for identifying dominant reactions in the ignition process. Difference in channels contributing ?H radicals causes a reduced global activation energy with decreasing temperatures.