Abstract: Calculation of the extinction properties in the infrared region by aerosol particles is conducted according to Mie light scattering theory . The calculated results show that there is an optimal particle size corresponding with the maximun extinction coefficient for a selected material at a given incident wavelength , and the size is smaller than the one related to the summit value of the extinction efficiency . The maximum extinction coefficients of several materials : silicon dioxide , carbon black , graphite and aluminium powder , have been evaluated as illustrations .