It is demonstrated by means of a MHD simulation that a finite thick velocity shear layer with super-Alfv¿nic velocity jump is unstable to the Kelvin-Helmholtz (KH) instability no matter how large the sonic Mach number, a result suggesting that the tail flank boundary is unstable to the KH instability. For supersonic shear flow the unstable mode becomes damped-oscillatory in the magnetosheath. For both subsonic and supersonic shear flows, the energy flux density into the magntosphere by the KH instability is large enough to replenish the plasma in the low latitude boundary layer with the tailward flow kinetic energy of observed intensity. A significant fraction of the energy flux density can reach deeper into the magnetosphere and its intensity is comparable to an energy flux density required for excitation of a ULF wave in the magnetosphere. ¿ American Geophysical Union 1990