This paper presents numerical simulations of two-dimensional laminar methane/air premixed jet flames with a detailed chemical kinetics mechanism. Comprehensive transport and thermodynamic physical properties for each species and gas mixture are included and the nonunity Lewis number has been taken into account for a nonuniform burning pattern of fuel-rich premixed flames. The focal point of this work is to demonstrate the sensitivity of the modelling of the detailed chemical kinetics on the flame temperature and concentrations of major components for three different equivalence ratios. The temperature and major species distributions are in good agreement with the experimental measurements by Nguyen et al., but the CO, OH and NO radical species profiles still deviate from experiments. The typical flame structure of the steady jet premixed methane/air flame has been obtained.