A method of evaluating the aerosol size distribution from the spectral attenuation measurements is shown. The process consists of solving the simultaneous integral equations, and examples are given of solutions based on the attenuation measurements made by Knestrick et al. over the Chesapeake Bay. It is found that the evaluated individual size distributions do not necessarily follow the power law, although departures from it are mostly small. If the power law is to be adopted neglecting small departures, the evaluated results are in average expressed by r(-3.57), where r is the radius of aerosols. In this study, the refractive index of aerosols is assumed to be 1.50, and some discussion is made of the effect of adopting a different refractive index value on estimation of the size distribution.