With high order Mie scattering theory, we studied the absorption (extinction) spectra, of colloidal CdS, ZnS microcrystallites, and the dependence of the apectra on crystallite size. It is predicted that the blue shifts of absorption peak and absorption edge are mainly caused by classical size effect at the microcrystallite radii of CdS, ZnS larger than 10 nm, otherwise, the blue shifts are mainly due to quantum size effect.