To accurately model and understand the mechanism of the evolution of microstructure during swelling, it is extremely important to model the mechanical response of smectite clay interlayers. The mechanical response of the interlayer of dry and hydrated montmorillonite is evaluated in this paper using steered molecular dynamics simulations. An atomic model of the sodium montmorillonite interlayer is first constructed. Dry montmorillonite and montmorillonite with one monolayer of interlayer water are studied. Using steered molecular dynamics, external forces can be applied to individual atoms to study the response of the model to applied forces. Displacement versus applied stress in the interlayer between clay layers for dry and hydrated conditions is evaluated by applying forces to the surface clay atoms. The paper describes the construction of the model, evaluation of the force field required for steered molecular dynamics simulations, the simulation procedure, and results of the simulations. The simulations not only provide the quantitative stress-deformation relationships but also give an insight into the molecular interactions taking place in dry and hydrated conditions. © 2007 NRC Canada.