Based on the NCEP/NCAR daily reanalysis data, the onset dates of the South China Sea summer monsoon(SCSSM) and Indian summer monsoon(ISM) are investigated, in such a way that the great and small difference years are defined by using the difference of the two onset dates. Results show that the difference has obvious interannual variations. In the small difference years, when the SCSSM bursts, the westerly has prevailed over Indian Peninsula, and therefore the two summer monsoons burst almost synchroneously. However in the great difference years, the ISM bursts(i. e. the westerly is prevailing over Indian Peninsula) in about the seveth pentad after the onset of the SCSSM, so the onset of the ISM is much later than that of the SCSSM. The analysis on mechanisms of monsoon onset shows that ISM onset is mainly related with the turning of meridional temperature gradient, while SCSSM onset is associated with the turning of zonal temperature gradient;the two onset dates are relatively independent from each other. The zonal temperature gradient turning is more important for the differences of the two onset dates. The two temperature gradient turnings depend on the appearance of the intensive belt of accumulated temperature contours.