Adsorption and desorption isotherm model of hardened cement paste under 20 °C is proposed. Adsorption and desorption isotherm of hardened cement paste with different water cement ratio, temperature history, and different degree of hydration were experimentally studied. Based on the experimental data, following characteristics were derived: 1) BET surface are of adsorption process and desorption process has linear relationship, 2) Relationship between statistical thickness of adsorption and relative humidity is identical in the range below 40%RH, 3) t-curve can be derived from the relationship between statistical thickness of adsorption and relative humidity, 4) difference between statistical thickness of adsorption and t-curve has a linear relationship between Kelvin radius, 5) virgin desorption curve value less than 40%RH has similar value of those of desorption curve after drying pre-treatment, 6) virgin desorption curve has a linear relationship with relative humidity. Based on these experimental points, empirical adsorption model is proposed.