A rheo-optical study on the colloidal alloys of binary mixture of silica spheres (110 and 300 nm in diameter) has been made in an exhaustively deionized suspension. The shear stress shows the yielding in the stress-strain measurements. The intersphere spacings increase first and reach a constant value as strain increases, whereas they decrease for the crystals of single-component spheres. The yielding phenomena in the stress-strain profiles were observed for all colloidal crystals of single-component spheres and colloidal alloys. The strain dependences on the peak wavelength and intensity in the reflection spectroscopy quite differ between colloidal crystals and alloys. The distortion of the colloidal alloys occurs at small shear stress and/or shear rate. The distortion of the electrical double layers, the sliding of the crystal planes, and the partial melting of the crystal structure are supported at rather high shear stresses and/or rates. Rheological properties in the steady-shear measurements of colloidal alloys are really crystal-like.