Abstract: The diffusivity of concrete is closely related to the microstructure which involves the interfacial transition zone (ITZ) between aggregate particles and cement paste at the meso-scale, as well as the microstructure of cement paste itself. At the micro-scale, the cement paste and the ITZ can be assumed to be composed of an intrinsic cement paste phase and a pore-water inclusion, with different porosities. Under external mechanical stress, the porosities and the connectivity of capillary pores within the cement paste and the ITZ will be changed, resulting in the change of the chloride diffusivity of concrete. In this study, a multi-scale analytical method that can predict the chloride diffusivity of concrete subjected to low-level stress is proposed. The quantitative relationship between the apparent diffusion coefficient of concrete and the mechanical stress (herein i.e. the volumetric strain) as well as the current porosities of cement paste and the ITZ is evaluated. The main parameters in the analytical method are the capillary porosities of cement paste and the ITZ, the mechanical properties of the intrinsic cement paste phase and the aggregate, the volume fractions of aggregate and the ITZ, and the mechanical stress. A good agreement between theoretical results and available test data illustrates the reliability and accuracy of the developed analytical approach. Based on the presented analytical method, the effect of micro- / meso-structure of concrete on its apparent diffusivity was investigated.