Sea bottom stress is conventionally assumed to be directly proportional to the square of the vertically averaged velocity

and the drag coefficient to be dependent on the speed and direction of the wind on the sea surface

the depth and dimension of the sea

the period of the tide and so on. In this paper a three-dimensional numerical model is used to discuss the relation between the drag coefficient and the above-mentioned factors. It can be shown from calculation that the relation

$$\left| {\overline {\tau _b } } \right|\infty \left| {\overline V } \right|$$ is valid

that the drag coefficient is a constant in a major part of a sea as thought conventionally

and that there is a small area near the coast where the drag coefficient is far greater. We call it singular area. A number of conclusions on the relation between the drag coefficient and the speed and direction of the wind

the sea depth and so on

were obtained.