Recycled aggregate concrete(RAC) specimens with different recycled coarse aggregate replacement ratios by mass(R) are fully immersed in a 10% chloride solution for 235 days. Both free chloride concentration(Cf) and total chloride concentration(Ct) are then measured employing a rapid chloride test(RCT) system. The Fick’s second diffusion law is verified by the test data and used to predict chloride concentration distribution at different immersion periods. Additionally, pore structures of the new and old mortar in RAC are tested by mercury intrusion porosimetry(MIP). It is found that binding chloride concentration(Cb) linearly increases with Cf. In natural aggregate concrete(NAC), Cf is obviously smaller than that in the RAC in the same testing environment and this is different from the test results of Ct and Cb. Furthermore, the effects of R on the chloride concentration of RAC become severe for specimens subjected to long immersion periods, which can be explained in terms of the pore structures of the new mortar and old adhesive mortar in RAC. Both free chloride concentration (Cf) and total chloride concentration (Ct) are then measured by mass (R) are fully immersed in 10% chloride solution for 235 days. The rapid diffusion test (RCT) system. The Fick’s second diffusion law is verified by the test data and used to predict the chloride concentration distribution at different immersion periods. Additionally, pore structures of the new and old mortar in RAC are tested by mercury intrusion porosimetry (MIP). It is found that binding chloride concentration (Cb) linearly increases with Cf. In natural aggregate concrete (NAC), Cf is obviously smaller than that in the RAC the the same testing environment and this is different from the test results of Ct and Cb. Furthermore, the effects of R on the chloride concentration of RAC become severe for specimens by to long immersion periods, which can be explained in terms of the pore structures of the new mortar and old adhesive mortar in RAC.