For investigating the effect of porosity in cement-based composite materials on diffusion coefficient of chloride ion, a series of mortar specimens with different water-cement ratios, different volume fractions and different diameters and graduation of aggregates are cast, and then chloride diffusivity of mortar specimens are measured by steady-state electric acceleration method and pore structure parameters of corresponding samples are obtained by mercury intrusion porosimetry. At the same time, the pore size distributions are visualized by X-ray micro-focus computer tomography. The results show that porous interfacial transition zones between aggregate and bulk cement paste significantly change the distribution of pore structure in hardened cement paste, and the smaller water-cement ratio is, the more significant the effect of interfacial transition zones on pore structures of hardened cement paste is. A good correlation between the total porosity, capillary porosity and the diffusion coefficient of chloride ion in cement-based composite materials has been determined and the continuous pore diameter is linearly related to the diffusion coefficient of those. Furthermore, the larger the continuous pore diameter is, the larger the value of chloride diffusivity is.