A method of calculation of an i.r. band shape suitable for overlapping lines has been proposed. Line mixing effects are considered using an impact model based on the Burshtein-Temkin theory which treats the rotational motion of molecules classically. The kernel of the integral equation defining spectral function is used to calculate the rotational relaxation matrix. For a linear-molecule-atom pair with repulsive interaction, the ovaloid-sphere collision model was found to be applicable for calculation of the angular velocity changing distribution. The parameters of the model are estimated using ab initio intermolecular potentials. The developed method is applied to CO and CO2 i.r. band shape calculations.