Abstract: This is a primary attempt at the composite analysis of the basic structure and evolutional features of tropical cyclones in South China Sea.The composite analysis shows that the flow fields of South China Sea cyclones arc roughly circular in shape with a radius of about 4 deg.lat., and accordingly pertain to the sub-synoptic scale. The cyclones extend vertically up to about 300mb with different orientation of the vorticity axis at different levels and different stages. During the severe storm stage,the votricity axis is nearly vertical and the radial inflow layer locates belowthe 500mb level, somewhat lower than that of western Pacific typhoons. The temperature of cyclone center at different stages is 2-6 ℃ higher than that of the surrounding air.The warm core is situated near the 300mb level while & cold center appears around 200mb. The value and elevation of either the warm or the cold center are lower than their counterparts of western pacific Ptyphoons respectively.Besides,the distributions of temperature and humidity clearly indicate that the heat and vapor supporting the development of tropical cyclones come chiefly from low levels to the south of the cyclone center.The paper further discussed the influence on the intensification of cyclones exerted by a number of factors, such as the middle/low level flow pattern in the environment, the warm core structure,the vertical motion,stability, etc.