The diffraction elastic constants are necessary for stress measurements of multi-phase materials by X-ray and neutron diffraction methods. The secondary phases can influence the diffraction elastic constants as well as the mechanical elastic constants. The micromechanical analysis based on Eshelby's inclusion mechanics was conducted to obtain the mean stress in diffracting crystalline phases in multi-phase composites under the applied stress. The diffraction elastic constants of multi-phase materials are determining as a function of the mechanical elastic constants. The method was applied to predict the effect of secondary phase on the diffraction elastic constants of sintered alumina with different purities. The predicted values for alumina 146 and 2.1.10 diffractions agree very well with the experimental results obtained by the X-ray diffraction method.