Tunable diode laser absorption spectroscopy (TDLAS) is a powerful technique to measure trace gas, which can provide high sensitivity, high selectivity, and fast time response. A brief description of our instruments with room-temperature near infrared tunable diode laser designed to measure greenhouse gas (i.e., CH4, CO2) in the ambient air is presented. A multiple-reflection cell and the second harmonic detection technique are used to lower the detection limit. The detection limit of the instrument is below 100 ppbv for CH4 and 10 ppmv for CO2, which is enough to the measurements of ambient CH4 and CO2. The instruments have been used to monitor the methane and carbon dioxide of the ambient air in a long time in Fengtai, Beijing. The results of measurement are shown and discussed in this paper.