This paper is a preliminary study on the hydrodynamical model describing the thermal stucture and the properties of circulation, related to the Cold Water-mass of yellow Sea. From analysis of observations we obtain following five characteristics: 1. The variation of temperature is larger than that of salt, the variation of density is, therefore, essentially dependent on the variation of temperature. The state equation of the sea water may be approximately written as ρ=ρo(1—aT). There is a thermocine layer which varies with season, and divides the Cold Water-mass into upper and lower homo- This paper is a preliminary study on the hydrodynamical model describing the thermal stucture and the properties of circulation, related to the Cold Water-mass of the Yellow Sea. From analysis of observations we can see the following five characteristics: 1. The variation of temperature is larger than that of salt, the variation of density is, therefore, essentially dependent on the variation of temperature. The state equation of the sea water may be approximately written as ρ = ρo (1-aT). There is a thermocine layer which varies with season , and divides the Cold Water-mass into upper and lower homo-