Abstract: SCSMEX assimilative analysis fields and NCAR interpolated outgoing longwave radiation (OLR) have been employed to describe large scale and synoptic features of subtropical circulation during the Bay of Bengal (BOB) monsoon and South China Sea(SCS) monsoon onsets in 1998.The physical linking between the BOB monsoon onset and SCS monsoon onset as well as the impacts of convective condensation heating associated with the BOB monsoon onset on the withdrawal of low level subtropical anticyclone is investigated. Results show that the Asian monsoon onset occurring during May 1998 exhibits a typical eastward development from the BOB region to the SCS domain. The vigorous convection over the BOB during and after the BOB monsoon onset results in the development of westerlies and convection over the north part of SCS. It is shown that the condensation heating is helpful in the overturning of the meridional gradient of temperature over the SCS. As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS.