Abstract: With 60 SXR FTIR spectroscopy and variable optical path cell, transmissivity measurements of trace gasses (CO2 and CH4) have been made. The main experimental results and its data processing are descripted. Using FASCODE 2 program, transmissivities of CO2 and CH4 which are under same conditions as experiments have been calculated. And, the calculated results are compared with the experimental results, respectively.