Abstract: Optical properties of compact internal-mixed aerosol particles composed of black carbon coated with sulfate were carried out utilizing discrete dipole approximation (DDA) and Bruggeman effective medium theory for the size parameters from 0.1 to 25. The validity of effective medium theory(EMT) in the light scattering of compact internal mixture was performed based on the comparison of both methods. For the monodisperse particles, the EMT does an adequate job of simulating the extinction-, absorption-, scattering-, backscattering-efficiency factors, asymmetry parameters, extinction-to-backscatter ratios and single scattering albedos in Rayleigh scattering region with all the relative differences less than 7%.Nevertheless, the EMT does not do an adequate job in the range of Mie scattering except for the cases that the inclusions of the internal mixture are small enough with the volume percentage less than 1%. Usage of the EMT can mainly lead to significant underestimations of the scattering efficiency factors but overestimations of absorption efficiency factors and asymmetry parameters when the size parameters are more than 4. For the polydisperse particles, the EMT does an adequate job for the optical parameters with the relative differences less than 9%.