The　chloride　diffusivity　in　cement-based　composite　materials　is　affected　by　multi-scale　pores,　including　gel　pores,　capillary　pores,　entrained　and　entrapped　voids,　micro-cracks,　etc.　The　pore-structure　parameters　(e.g.　porosity)　will　change　when　subjected　to　external　loadings,　resulting　in　the　change　of　the　chloride　diffusivity　in　cement　paste.　The　effect　of　the　external　loadings　on　the　chloride　diffusivity　can　be　assumed　as　the　change　of　porosity　on　the　chloride　diffusivity　induced　by　external　loadings.　In　the　present　study,　saturated　cement　paste　is　regarded　as　a　two-phase　composite　composed　of　instinct　cement　matrix　(with　zero　porosity)　and　pore-water　inclusion.　Based　on　the　theory　of　elasticity,　the　quantitative　relationship　between　current　porosity　of　mortar　and　initial　porosity　as　well　as　volumetric　strain　before　reaching　the　strength　of　mortar　(i.e.　before　the　appearance　of　new　cracks)　is　deduced.　Moreover,　the　quantitative　relationship　between　the　chloride　diffusion　coefficient　of　cement　paste　and　these　parameters　is　obtained.　The　effects　of　external　loadings　(herein　i.e.　the　volumetric　strain)　and　porosity　variation　on　the　diffusion　behavior　of　cement　paste　are　investigated　on　the　basis　of　the　Fick＇s　second　law.　It　is　found　that　the　chloride　diffusion　coefficient　in　saturated　cement　paste　increases　significantly　with　the　increasing　of　initial　porosity.　Furthermore,　the　diffusion　coefficient　decreases　with　the　increase　of　compressive　strain,　while　the　diffusion　coefficient　increases　with　the　increase　of　tensile　strain.　©,　2015,　Tsinghua　University.　All　right　reserved.