We perform a numerical study of the effect of including a small amount of hydrogen in lean methane-air premixed flames at high pressure and high temperature conditions. It has been shown recently (Bell and Gupta (1997)) that hydrogen addition extends the lean operating limit of natural gas engines, leading to a potential decrease in pollutant formation. We suggest here that the origin of this effect is that, at constant global equivalence ratio, the stretch resistance of these flames is considerably increased by hydrogen blending, while other dame properties, such as ignition time and burnt gas temperature, are comparatively little modified.