By measuring the permeability of mortar specimens with an oxygen diffusion equipment, the open porosity of hardened cement paste(hcp) was evaluated and the durability of concrete was discussed in this research. The research shows that the results of measuring oxygen diffusion can correctly reflect and reveal the effects of some factors such as water cement ratio, composition of cementitious materials, curing condition on the pore structure and permeability of hcp. There is a good relativity between the permeability and the sulfate resistance of mortar.