A model is presented for describing the membrane potential-dependent motility of the outer hair cell. This model assumes that the motility is due to conformational changes of motor molecules in the plasma membrane. Two kinds of experimental observations, elasticity of the cell and stretch dependence of the motor molecule, are important for characterize this motility by the model. The motor molecule can be described by a two-state model which has electrical and mechanical components in the free energy. The electrical component is due to the charge transferred across the membrane and the mechanical component is due to a change in membrane area in the two states. It can be shown that the elastic element and the motor element are connected in series. Thus the apparent strain of the cell is represented by the sum of true elastic strain and changes due to motor molecules. This model predicts the amplitude of the movement and the force produced by the motility. The model predicts the force produced under isometric condition is about 0.1 nN/mV, in agreement with values estimated from in vivo conditions. The effect of an elastic load attached to the cell is also discussed.