Characteristics of vertical components of 574 strong-motion accelerograms were studied in order to know the effects of vertical ground motion on dynamic stability of structures. The followings were made clear.
　1) According to accelerograms of the maximum acceleration more than 50gals,the ratio of the maximum acceleleration of vertical component to that of horizontal component was less than 1/2.
　2) The occurence time of the maximum acceleration of horizontal component was different from that of vertical component. At the time that the safety factors of sliding and overturning were smallest, the ratio of vertical acceleration to horizontal acceleration was less that 1/3.
　3) The stability of the gravity type quay walls were analyzed taking account of the vertical seismic coefficient according to the current design method of port facilities. As a result of this design calculation, the vertical seismic coefficient had little influence on the safety factor of a gravity type quay wall.