In recent research works on capsizing in beam sea conditions it is reported by some authors that the model ships were capsized by breaking waves which gave them a large overturning moment due to the direct attack of high speed wave fronts. But the model shapes were different from the ones of small ships those are mainly reported in the record of maritime accidents to have capsized. This paper deals with the capsizing of a small fishing boat and shows the different patterns of capsizing phenomena from other papers. Many capsizing could be seen in concentric transient waves with two successive crests, which excited a large rolling amplitude beyond the stability vanishing angle of the vessel. The model ship was of about 1/6 scale of an actual fishing boat and set free-floating abeam in the model basin. The GM values were varied in a wide range so that critical GM values could be determined. The effect of the initial heeling angle and the initial position of the model was also investigated. The maximum wave height was about the same order as the model breadth. As for the mechanism of capsizing it is concluded that the model ship was capsized not by the impulsive moment of breaking waves directly but the loss of stability due to a large rolling motion developed by the transient large waves. The breakers played a significant roll to change the conditions of encountering the next critical waves.