A new method is proposed for the measurement of imaginary part of refractive index n i of aerosol by using an optical particle counter (OPC) and a visibility meter (VM). The number density distribution of aerosol particle is measured by an OPC with nominal radii, and visibility is measured simultaneously with a VM. According to the Mie scattering theory, the different distributions can be obtained with different n i for the same set of data measured by OPC. Then, a curve of extinction coefficient can be calculated with n i from these distributions. Actual value of n i can be decided with the real extinction coefficient provided by visibility measurement. Finally, the actual number density distribution of aerosol particles is obtained from the OPC data and the decided ni.