A tunable diode laser absorption spectroscopy system with a multi pass cell and a DFB laser diode at wavelength 1.626 μm has been developed for simultaneous measurement of ethylene and methane. A line separation method based on multi absorption peak using the least squares algorithm has been established. The characteristic peak positions and relative intensities of ethylene and methane were determined by available spectral structures from previous investigations and available databases. The accuracy of the measured concentration is within 5% comparing with the mass flow meter.