Recently, there are many researches on sea level rise due to global warming. Various problems on aquatic environment and disaster prevention will be caused by the sea level rise. However, variations of high water level (H.W.L.) and low water level (L.W.L.) are important to discuss countermeasures against these problems rather than that of mean sea level (M.S.L.). Changes in H.W.L. and L.W.L are determined by a combination of variability in M.S.L. and tidal amplitude. We tried to clarify long term global changes of oceanic tidal amplitudes by usung the harmonic analysis. From the results of this study, we found that tidal amplitudes changed during 20th century. The trends were increase in most part of the world.
