Abstract: CaO clinkers of different compactnesses were carbonated at 700°C in CO2 atmosphere with the carbonation rate controlled by diffusion process, regardless of the difference between compactnesses. For the clinker of low compactness, the carbonation reaction occurred simultaneously both on surface and inside pores, and each CaO grain was surrounded by CaCO3 thus formed, in which microfissures were found. But for the high-compactness samples, carbonation reaction only occurred on surface, with a compact CaCO3 layer thus formed which was combined well with the substrate material. Hydration test result showed that the clinkers after carbonation could effectively improve the hydration resistance of the CaO clinkers of either low or high compactness. However, the carbonated low-compactness clinker is prone to breakage during hydration process. It is therefore concluded that only the compact CaO clinker with low apparent porosity is suitable to carbonation for improving its hydration resistance.