The formation of calcium silicate hydrate (C-S-H) and its carbonation were investigated under two different atmospheric conditions. The C-S-H was prepared with calcium hydroxide solution and tetraethyl orthosilicate to have different Ca/Si mole ratios. With rising initial Ca/Si mole ratio, the amount of C-S-H increased and that of co-existing silica gel decreased under the isolated condition. The Ca/Si mole ratios of formed C-S-H have the approximately constant value of 1.2 and its mean solubility product is 5.5x10^-49 at room temperature. Under exposure to CO₂, the C-S-H formed in the liquid decomposes by carbonation and then forms calcite and silica gel. This carbonation process of C-S-H could be divided into four stages: (I) Formation of C-S-H, calcite and the Ca(II)-modified silica gel, (II) Formation of calcite from the residual calcium ion, (III) Decomposition of C-S-H and release of silicate ion, (IV) Change from the modified silica gel to the pure silica gel.