Abstract: The tropical Indian Ocean subsurface temperature anomaly above 400 m is studied by using the method of EOF and other statistic methods, and the 1997/98 tropical Indian Ocean dipole event is analyzed.The results show that the largest temperature anomaly in the tropical Indian Ocean subsurface layer above 400 m is around the depth of 100 m.As for the whole tropical Indian Ocean, the subsurface layer above 60 m appears to have a warming trend since the 1980' s, while the subsurface layer below 80 m to have a cooling trend.The subsurface layer of 80 m has an obvious dipole mode, which is much stronger than that of the sea surface.In 1997/98, the dipole event of the sea surface in the Indian Ocean was resulted from the westward ex tension of the temperature anomaly over the eastern Indian Ocean.The westward ex tension of the temperature anomaly was related to the easterly anomaly over the tropical Indian Ocean, where the easterly anomaly might trigger the Rossby wave and induce the temperature anomaly to ex tend westward.