It is obvious that the up-set of sailing boat arises when the heeling angle of them on account of wind force exceeds the angle of vanishing stability in its magnitude. In this paper, the author, relying on some assumption, studied about the righting moment of the boats, and the wind-pressure acting in transverse direction and trying to heel over the boat, and then computed the maximum angles of inclination in various wind and boat conditions. In the result, it was understood that the heeling angles increased remarkably, in the same speed of wind, when the boat was sailing at a low speed, or when the wind force changed suddenly, and that some characteristics of heeling angles lied.