Abstract:In nature there are many phenomena having the temporal characteristics of multipled nine days, such as the period of nine day in the Earth's rotation, in the solar and lunar tidal forces, in geomagnetic activity, in atmospheric rhythm and so on. In the case, strong earthquake occurrence time may be modulated and triggered by the phenomena, so earthquake occurrence in some degree is characterized by the temporal regularity of multipled nine days.It is valuable for impending earthquake prediction.Based on occurrence dates of strong earthquakes in Gansu Province and neighbour regions the objectivity of multipled nine days have been demonstrated.