Abstract: Fault or crack must all undergo a quasistatic accelerating extension process before the instability occurs.The acceleration is a necessary condition of instability.The accelerating release is an universal precursory of a great earthquake which nucleates within the brittle seismogenic layer.This acceleratig process can be summarized briefly as the cumulative seismic releasing rate to be proportional to the in-verse power of the remaining time to failure.Based on this principle, the foreshock sequences of the Xingtai M_s 7.2 earthquake, Hebei Province, on March 22 in 1966 are analysed.It is shown clearly that the time—to—failure and magnitudes of the large foreshocks in the sequence, especially the main shock,can be predicted successfully by linearized regression, but the maximum coefficient r must be obtained.In the estimates of main—shock time, making it in the developing sequences, the errors can be less than one—half the time remaining between the time of the last data point used and the main shock or far less.