Abstract: The tidal stress field of a layered spherically symmetric Earth due to the Sun and Moon is used to investigate its relation with earthquake occurrence. For the 70 strong earthquakes occurred during the last 24 years in or near China, we computed the tidal stress in spherical polar co-ordinate at the focus at the time of occurrence. The normal and tangential stress components along the slip direction on the earthquake fault plane are then found by co-ordinate transformation. Coulomb's criterion of shear fracture is used to determine whether these stress components will trigger the earthquakes or not.The results show that among the 70 earthquakes, 43 have had triggering effect and for the 18 events occured in North China 14 of them have had triggering effect. It may be concluded that for shallow strike slip earthquakes there are significant triggering effect, while for oblique slip and dip slip earthquakes tidal stress has very little triggering effect. From 72 events occurred over other parts of the world similar conclusion is obtained.Finally, for the purpose, of earthquake prediction, a way in finding a time interval during which the tidal stress may trigger earthquake on possible faults in North China is suggested.