The daily NCEP/NCAR reanalyzed data and daily rainfall data from National Climate Center of China Meteorology Administrator for the period of 1958-2000 were employed to diagnose the dynamic and thermodynamic characteristics associated with the onset and retreat of South China Sea summer monsoon

as well as the differences compared with South Asian monsoon on climatological mean. Possible impacts of precipitation in the yearly first raining period in South China on the onset of South China Sea summer monsoon are explored. The conclusions are as follow: (1) The allocation of atmospheric dynamic and thermodynamic characteristics over the South China Sea is distinctively different from that in the Bay of Bengal and South Asia during seasonal transitions. Tropospheric temperature meridional gradients under 850 hPa reverse (from negative to positive) later than the onset of southwest monsoon in South China Sea. (2) Under the constraint of thermal wind relation

850hPa westerly onsets when the tropospheric temperature meridional gradient at 925hPa and 1000hPa is still negative. (3) During seasonal transitions

the allocation of atmospheric dynamic and thermodynamic characteristics over the South China Sea is distinctive owing to the unique geographic position and the impact of cold air from middle latitudes. (4)With reinforcing rainfall in South China

convection latent heat on the middleupper troposphere favors the reverse of the temperature meridional gradient. South Asia high in the upper troposphere consequently moves northward under the constraint of thermal wind relation. Therefore

South China frontal raining in the yearly first raining period is a beneficial factor for the onset of South China Sea summer monsoon.