In the present paper, a review and analysis of the theory of direct absorption spectroscopy and modulation spectroscopy using tunable diode laser were provided, and a remote sensing system of CH4 was designed. A near infrared tunable diode laser operating at wavelength 1.65 microm at room temperature was used to perform the measurement of CH4, using the R(6) line of, the 2upsilon3 band of methane. Signal to noise ratio and fundamental noise sources were also discussed. This work has laid the foundation for remote sensing of CH4 on-line.