Based on 1980-1997 T_BB data from the GMS of Japan and the NCEP/NCAR daily reanalysis data(1958-1997),an investigation is carried out of characteristics of convection activities over Sumatra area and its relationship with the South China Sea summer monsoon(SCSSM) onset.The results show that the onset of the SCSSM bears a close relation to the systematic moving of the convection center over Sumatra along the "land bridge" and Indo-China Peninsula from winter to summer.The sudden acceteration of convection shift affects directly the subsequent enhancement of the convection over Indo-China Peninsula area followed by the rupture of the subtropical high band in this region,leading to the SCSSM establishment.The convection exhibits a dipole distributive pattern between the SCS and Sumatra area before the SCSSM onset and the convection change shows an opposite trend between the SCS area and the south of Sumatra area.Before(during) the SCSSM sets up, the convection over the south of Sumatra area thrives (weakens).Therefore,we can regard the beginning date of the time period when the T_BB difference between the SCS and the south of Sumatra area≤5K and maintains for one pentad as an index of the SCSSM onset.