Molecular dynamics (MD) simulations have been performed to study the flexibility of smectite clay minerals. We aim at the quantitative understanding of the mechanical behavior of a single clay layer in a completely exfoliated state. The repeating unit of a clay layer is taken to be a(0) = 0.52 nm and b(0) = 0.902 nm with formula of 2Na(1/3) Al-2[Si11/3Al1/3]O-10(OH)(2) which corresponds to that of beidellite. When the size of the basic cell (A = 9.3 nm, B = 2.6 nm, and C = 5 nm) (denoted by A-type cell) is reduced by 3-40% in the A-direction, the stationary structure of a clay layer is obtained as a curved sheet with a 2:1 smectite-type layer structure. In such a curved state, the layer experiences a stress of 0.5-0.7 GPa. The layer structure of a clay fractures when the size of the same basic cell is reduced by more than 40%. The bending constant is estimated for a curved layer by plotting the inverse of the average radius against stress. The similar calculations are performed by reducing the size of the basic cell (A = 3.1 nm, B = 10.7 nm, and C = 5 nm) (denoted by B-type cell) in the B-direction. The clay layer is found to be more flexible along the A-axis direction than along the B-axis direction. When the microscopic structure of a curved clay layer is examined, it is concluded that the main origin of flexibility lies in the change of Si-O-Si angles in the silicate tetrahedral sheets rather than in the change of bond lengths.