A technique was presented, which deduces the optical constants (the complex refractive indices) of small particles from the measured transmittance spectrum of a dilute polydispersion,by use of the precise Mie scattering theory and the Kramers Kronig dispersion relation. The multi valued characters and the suitable range of applications were discussed. The influence of the size distribution and the transmittance was thoroughly investigated,and it indicates that the errors of the size distribution and the transmittance will influence seriously the absorption index k, but weakly the refractive index n.