The driving force of drying shrinkage of hardened cement paste has been attributed to RTln(h)/v with external relative humidity h and water molar volume v in theories of capillary tension and disjoining pressure. However, these theories fail to explain the considerable hysteresis observed in length-change isotherms. In this study, the sorption isotherm and length-change isotherm of cement pastes were determined with different water-to-cement ratios and cement types, and internal pressure of shrinkage ∏ was calculated using the measured strain and elastic modulus of the skeleton. This internal pressure is a kind of disjoining pressure originated from hydration force and built up within adsorbed water films as a result of interactions between the hydrophilic solid surface and water molecules. Changes in internal surface energies of hardened cement pastes due to hydration, drying and temperature history result in a different statistical thickness of the adsorbed water layer under the same equilibrium relative humidity. The proposed model gives a rational explanation for the hysteresis in length-change isotherm.