The potential and current density distribution in a short-channel MOS transistor in saturation has been studied in detail by numerical simulation. The pushing effect of carriers from the surface in the draincontrolled region is discussed by introducing a field depth parameter. The pinch-off condition of the channel is examined: at gate voltages below a transition voltageVtr, the length of the pinch-off region is determined by the pushing of carriers away from the surface; whereas at higher gate voltages, it is determined by the velocity saturation of carriers. An analytical formula for the length of the pinch-off region at gate voltages belowVtris derived based on simulation results.