Environmental Scanning Electron Microscope (ESEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to in situ observation microstructure character changes of hardened cement paste due to carbonation. Three types water-to-cement ratio of 0.53, 0.35 and 0.23 were research. When w/c=0.53, Carbonation enlarged the cracks, and some calcium carbonate spots appear on the calcium hydroxide crystals. When w/c=0.35, less cracks appear, and some white flocculent calcium carbonated appear on the calcium hydroxide crystals than above w/c=0.53 cement paste. When w/c=0.23, the microstructure character before and after carbonation have distinct changes, and some cracks become small or disappeared. Unhydrated cement clinkers continue to hydrate reactions and there are many white SiO2•nH2O (silica gel) after C-S-H gel carbonation in cement paste matrix.