Key parameters governing an accurate concentration measurement of gases based on FTIR spectrum analysis are discussed. It is found that characteristic infrared (IR) absorption bands of the gas measurement of interest have to be completely free from those of other components. It is sometimes necessary to use several characteristic IR absorption bands to perform gas concentration calibration and measurement. Particular care has to be taken in FTIR spectrum in the case of high concentration of CO2 or H2O in the sample gas because CO2 and H2O possess strong IR absorption that N2O or NO, or NO2 peaks can be mistakenly overlooked. If ambient CO2 or H2O bands do not overlap completely with the peaks of measurement of interest, we can ignore their peaks and still be able to get accurate results with the other gas components of interest without purging the FTIR pathway. Several examples are provided. It is revealed that the concentration of NO, NO2, N2O, CO and CO2 in their mixture can be measured accurately using FTIR spectroscopy if procedures and concerns addressed are carefully followed.