Tunable diode laser absorption spectroscopy (TDLAS) is a new trace gas detective method developed with the combination of a diode laser source and a long absorption path. It has the significant advantage not only in sensitivity but also in rapidity of response. The concentration of the ambient methane has been measured using a TDLAS system developed in the laboratory. The system has some advantage s, such as sensitivity, low detective limit (below 0.087mg/m33) an d rapidity of response, and also it is easy to be integrated to a portable apparatus to mon itor the methane concentration in the atmosphere.