Based on OISST, ERSST and NCEP2 monthly surface wind data, the structure characteristics and formation of Southern Indian Ocean Dipole(SIOD) and its correlation with ENSO are investigated.Results show that SIOD develops from October to December, peaks in following February, and decays from April to June.Wind, latent heat flux and shortwave radiation fluxes play an important role in the formation of SIOD.The change of the mixed layer depth also contributes to the formation of SIOD.In recent three decades, SIOD are phase-locked to the austral summer season, preceding the ENSO signal by nine-to-ten months.Furthermore, the relationship between SIOD and ENSO is enhanced after interdecadal abrupt climate change in the mid-1970s.El Niño events usually occur after positive SIOD and La Niña events occur after negative SIOD.