Abstract: Two sets of numerical experiments were performed using a 5-layer limited area primitive e-quations model. One simulated East Asia Summer monsoon circulation with real land-sea distributions and the other with artificial ones. Their results were compared.It is concluded that the generation of 2 monsoon troughs, 3 SW monsoon cores and low-level cross-equatorial flows is closely associated with the east-west land-sea alignment in South Asia, for the absence of the Indian and Indo-china Peninsulas would result in the disappearance of the troughs and the reduction of the cores from 3 to 1 ; the intensity of the cross-equatorial flows east of Somali would significantly drop and the corresponding low-level jet would go should there be no the African continent; the Tibetan Plateau increases near-surface monsoon in South Asia and southern China substantially due to its geographic location, in that the enhancement of monsoon would weaken if the Arabian Sea and the Bay of Bengal were masses of land instead of ocean and contrarily, it would be larger if the 2 peninsulas were ocean rather than land.