The effect of steam addition on the structure and extinction of hydrogen/air premixed flames is investigated numerically with a detailed kinetic mechanism by adopting both counterflow and stagnation-point flow configurations as model problems. For a specified equivalence ratio of hydrogen/air mixture, the flames in a stagnation-point flow can be maintained to higher steam content compared to those in a counterflow. This can be explained based on the local strain rates defined at the location of maximum heat release rate. The effects of preferential diffusion and reaction incompleteness caused by (lame stretch and the effect of steam addition on flame extinction can be successfully quantified by adopting a local equilibrium temperature, which can be determined from chemical equilibrium calculation using the temperature and concentrations of a local mixture in a flow field. The structure of near-stoichiometric flames with the variation in steam content can be successfully explained by introducing a local equivalence ratio.