The problem on course stability of a towed vessel is of importance and interest for the ship design as well as safety of navigation. The stability of towed vessel will be affected by some factors such as the size of towed vessel or the length of tow line. Specially the wind effect is the most important factor in towing. In this paper, the relationship between the course stability of towed vessel and wind direction and wind speed are discussed by the theoretical way. The results on a cargo ship and a ferry boat as towed vessel are as follows. The stability of towed vessel under wind pressure depends on the inherent stability of towed vessel. However, the ship with large exposed area to wind such as a ferry boat is greatly affected by wind direction and velocity even if the inherent stability is stable. In general, the towed vessel will be much unstable for the range from beam to quarter wind in higher wind speed than ship speed. The length of tow line has also great infuluence upon the stability, long tow line will be able to stabilize for lower wind speed than ship speed.