Abstract: Adopting the outgoing longwavc radiation (OLR) and wind data of NCEP and the precipitation data of Japan,and using the filtering summer monsoon index(Im)in 30-60 d averaged over the area of the Bay of Bengal(BOB) and the area of the South China Sea (SCS) respectively to present the Intra-Seasonal Oscillation(IS0) of the Southwest Monsoon(SM) of BOB and the South China Sea Summer Monsoon (SCSSM),the 7S0 features of the two monsoons are compared.The ISOs of SM of BOB and of SCSSM have 3 and half times of ISO waves during the summer monsoon period (May-Oetober).Along the zone of the Arabian Sea-West Pacific, there are 4 times of summer monsoon ISO eastward propagation from Arabian Sea and 3 times of westward propagation from the western Pacific.After July, the westward propagation of ISO is inhabited and the cast-ward propagation can arrive to the western Pacific.(hhere are 4 times of meridional propagation of monsoon ISO in both the areas of BOB and of SCS during the monsoon period.In the area of BOB, it is northward propagation from the tropic eastern Indian Ocean south of around 15°N, and is southward propagation of sub-tropical monsoon ISO north of around 15°N.However, in SCS, it is 4 times of northward propagation of the monsoon ISO from the Tropic.Both in the respective 6 phases of ISO of SM of BOB and of IS0 of SCSSM,there is opposite-phase feature of the LF convection and circulation in the phase 1-3 to that in phase 4-6,which is caused by eastward and northward movement of IS0 in the tropic eastern Indian Ocean.The LF convcction propagates along the southwest-northeast direction from the tropic eastern Indian Ocean to BOB (needing about 14 d),exciting the ISO of SM of BOB, and then propagates eastward to SCS (needing about 6 d) to excite the ISO of SCSSM.It takes about 25 d for the ISO of SCSSM to propagate to South China with the result that the meridional and tonal relay propagation of ISO from the tropic Indian Ocean to South China experiences about 45 d.(hhe precipitation belt impacted by SM of BOB moves eastward with the LF convection south of 20°N,but the precipitation belt impacted by SCSSM,in addition to the above,also moves northward with the LF convection of SCS north of 20°N.