In order to improve the stability and to increase the working area for fishing vessel, bulges are often added on ship sides from underwater part up to upper deck. From the view point of seakeeping quality, it is important to clarify the effect of bulges on ship dynamic response as well as static stability. A fisheries research vessel “Ushio-maru” has been rebuilt (elongated and bulges fitted) recently. In this paper, we present the results of investigation into its stability improvement and study on the influence of bulge on its motion in waves by model experiments and theoretical calculations. It was shown that stability improvement is considerably achieved by fitting bulges. In addition, resonant roll motion was slightly reduced due to damping effect of bulges. It was also found that there are no significant difference in heave and pitch due to the existence of bulge. On the other hand, roll motion in beam and quartering long waves increase. Furthermore, at-sea seakeeping tests of the ship with bulges were conducted, motion characteristics in real ocean waves is described. Finally, authors point out the merits and demerits of fitting bulge to fishing vessel.