We have measured the magnetic hysteresis loops of an epitaxialYBa2Cu3O7−&dgr;thin film using our recently developed device which can provide the field magnitude in the range of 0–1000 Oe and the field sweep rate up to107 Oe/s.The shape of the hysteresis loop measured changes with the field-sweep rate up to the critical sweep rate; and over the critical sweep rate the ac magnetization reaches its real critical state where magnetization does not change even with a further increase in the field-sweep rate. The critical sweep rate is about106 Oe/sat 77 K. With the hysteresis loops and Bean model, we have calculated the magnetization critical current density(Jcac)which is consistent with that obtained byI–Vmeasurements. We have also studied flux motion and activation energy under the high sweep rate magnetic field. At temperature 77 K, the velocity of the flux motion is of the order 10 m/s and the pinning energyU0/kis about 339 K which is much smaller than the magnetization decay measurement. ©1997 American Institute of Physics.