A high resolution line-by-line model to calculate the molecular gas spectra is developed. With the model, the absorption coefficients can be calculated using molecular data from the up-to-date spectroscopic databanks (HITRAN2008/HITEMP2010). The model can choose the proper databank according to the temperature condition. Thus, it can calculate the absorption coefficient on high temperature and room temperature. The sampling interval is set constant. The choice of interval depends on the factor that the resolution must be high enough to resolve the typical gas line fully. The model uses equal line wing cutoff to take account of contribution of the most important spectra lines. The total internal partition function for the spectra lines is calculated using the tertiary polynomial expression which is fitted by Gamache. The spectra line profile is determined according to the gas temperature and pressure. Finally, the model is employed to calculate the spectral transmissivity of CO2 at various temperatures, concentrations, and path lengths for the 4.3 μm and 2.7 μm bands. Then the line-by-line calculated data is used to calculate narrow band transmissivity at the measured resolution, accounting for instrument broadening by the FTIR. A comparison between the narrow band and measured transmissivity showed a good agreement among a wide temperature area.