Abstract: Data of sea level changes corrected for the influences of periodic and non-geriodic fac tors show relative crustal deformation in the areas of tide gause stations. By analysing these data for three large earthquakes occurred in the Bohai bay-area, it may be seen that the variations of sea level before large earthquakes are fairly obvious. They appear to be significant fod medium-term prediction of larg earthquakes and also give precursory information before moderate and small earthquakes. This paper gives successful examples in predicting certain earthquakes.