The extinction limits of methane-air diffusion flames in preheated air are studied experimentally and theoretically. The air temperature is varied in the wide range from ambient temperature (285 K) to as high as 1500 K. The fuel employed is methane with initial temperature of 300 K, Fuel or air diluted with Ar, N-2, or CO2. For all conditions studied here, it is found that extinction of the diffusion flame occurs up to 1500 K, which is significantly above the autoignition temperature, The experimental results can be well reproduced in terms of pre-exponential factor and activation temperature of a hypothetical one-step overall reaction as determined by application of activation energy asymptotics.