The effect of spectral line mixing on atmospheric transmission functions at tangential paths in the region of carbon dioxide absorption bands is studied. Two calculation methods are used to consider this effect: the method based on the Rosenkranz profile and the method of varied interactions between branches of spectral bands, which has recently been proposed by the authors of this work. A comparison between transmission functions calculated with and without considering the mixing effect shows its significant (up to 20-30%) influence on atmospheric transmittance in the region of the near wings of lines. The transmission functions calculated with the line mixing effect included agree with those measured in the ATMOS experiment to within 2-5%. The discrepancy (up to 10%) between the values of the effect predicted by two calculation methods are noted and the reasons for this discrepancy are discussed.