Corrosion of steel bars in concrete structures is initiated as a result of concrete carbonation and/or chloride intrusion, and influenced by their interaction. This paper presents an experimental investigation into the effect of chloride ions on carbonation of cement paste by means of X-ray CT techniques and mercury intrusion porosimetry(MIP), which is benchmarked by the conventional phenolphthalein method. A group of the cement paste cylinders with different amounts of chlorides ions were manufactured and cured before they were subjected to an accelerated carbonation process in a conditional cabinet regime for different ages. The carbonation front of the cement paste was first evaluated using phenolphthalein method. This was followed by an investigation of microstructure evolution of the cement paste using XCT and MIP techniques. The experimental results show that the carbonation of a cement paste increases with its water to cement ratio and with carbonation ages, but decrease with its amount of chloride ions. In particular, it has been found that increases of chloride ion of a cement paste refine its porous structures, decrease its porosity and eventually mitigate its carbonation rate. The relevant results can be referred to for durability design and prediction of reinforced concrete structures.