Abstract: The spatial and temporal feat ures of the multi yearly pent ad mean 850h Pa wind fields and the TBB fields were analyzed. It was found that the tropical Asian summer monsoon (TASM) onsets simultaneously and abruptly first over the wholearea in the Bay of Bengal (BB), the Indo-China Peninsula (ICP) and the South China Sea (SCS), east of 90°E, in the 27th to 28th pent ads of a year (pentads 3 to 4 in May) and therefore the whole area is the source area of the TASM;while the tropical Indian summer monsoon (TISM) onsets f irst in the south part of the India Peninsula (IP) and the Arabian Sea (AS) in the 30th to 31st pent ads. Then it advances gradually northward and onsets in the whole area w est of 90°E, by the end of June. Possible mechanisms were carefully analyzed and it was found that the rapid seasonal variations of the surf ace sensible heat flux, the air temperature and the surface pressure in spring and early summer in the middle to high latitudes of the East Asian continent between 110°E and 120°E is crucial responsible to the earliest onset of the South China Sea summer monsoon (SCSSM). It is the rapid warming that induces a continental depression to form and break throug h the high system of pressure originally located in the above continental areas. The low depression in turn introduces the south westerly to come int o the BB to the SCS areas, east of 90°E, and thus makes the SCSSM burst out first in Asia. In the IP area, west of 90°E, the surface sensible heat flux does not change almost during Apr. and M ay, the T ISM onset s lat er than the SCSSM by about 1 month. Therefore, it is concluded that the meridian of 90°E is the demarcation line between the South Asian summer monsoon (SASM, i. e. the TISM) and the East Asian summer monsoon (EASM, including the SCSSM). The relations of the onset time of the tropical summer monsoon to the seasonal changes of the South Asian High (SAH) at the 200 h Pa level are emphatically discussed. It was found that the SAH center position has close relation to the onset time of the tropical monsoon. When the SAH center crosses 20°N, the SCS summer monsoon onsets and when it crosses 25°N the Indian summer monsoon begins in the south part. By comparison of the sodetermined onset time with that determined by other methods, it is seen that they are in good consis tence in the SCS area, however, they are some what different in the IP area.