A mixture-based constitutive model was developed on the basis of the real mixture status of nanocrystalline materials to describe the mechanical properties of the materials. The nanocrystalline materials are composed of grain and grain boundary phases,and the grain boundary phases are assumed to have equivalent parts: part I and part Ⅱ. The strain of the part I is the same as that of the grain interior; meanwhile,the stress of the part Ⅱ is equals to the total stress of the grain and part I of the grain boundary,which is well in agreement with practical ones. Then the established model was applied to calculate the elastic modulus of porous nanocrystalline materials. Furthermore,the model is extended to describe the stress-strain relation with small plastic deformation. The model can predict the effects of grain size and porosity on the elastic modulus and yield strength of the nanocrystalline materials. The simulated results are well in agreement with experimental ones in small plastic strain range.