Based on OPAC refractive index database and lognormal distribution, the radiative properties of dust aerosols and global averaged direct radiative forcing are studied by using Mie theory and a simple concept model. The relationship between mode radius and extinction efficiency, scattering efficiency and absorbing efficiency of single dust aerosols are analyzed too. We also discuss the change of optical parameters of the dust aerosols with the wavelength. The result shows that extinction efficiency and scattering efficiency oscillate and decrease as the mode radius increases. However, the oscillation gradually disappears as the wavelength increases. Extinction coefficient is substantial sensitive to mode radius, while single scattering albedo and asymmetry factor are opposite. Single scattering albedo of dust aerosol can strongly influence on the globally averaged direct radiative forcing. With mode radius increases, the negative globally averaged direct radiative forcing decreases, but the positive radiative forcing increases.