The structure of premixed laminar methane/air flames at atmospheric pressure was studied in this paper. In the post-flame region of these flat flames, the emission specctroscopy was used to obtain the temperature and gas species concentrations information which are very important in the study of combustion. Based on the thermal emission by gas molecules in lean flames, the emission data obtained by using the multiwavelength infrared detection method were operated in an iterative algorithm to get the gas species moles fractions and temperatures. At the same positions in the flames, temperature measurements were also obtained from thermocouple and H2O and CO2 concentrations were obtained from gas chromatography. The emission spectroscopy provided accurate temperature and mole fractions data compared to the theoretical value with the consideration of reasonable heat loss.