Abstract: A discussion on the observed fact that the frequency of aftershocks of the 1966 Xingtai Earthquake increased before the occurrences in North China of a number of strong earthquakes subsequent to 1966, was made. Thus the activity of the Xingtai aftershocks may be considered as a "window" to monitor the increase of tectonic stress, taken North China as a whole and the rise of the frequency of Xingtai aftershocks, a precursory phenomenon for North China earthquakes of greater magnitudes.