To reduce a roll motion of the 5.1k LNG bunkering vessel with IMO type C tanks, a new cylindrical tank is developed. The coupling effect between the roll motion of vessel and inner-tank flow is investigated by a series 1:100 scale model tests in two-dimensional basin. The vessel fitted with two cylindrical tanks was designed and built. The series of model tests with the vessel in five different load conditions are conducted, namely Empty(0%), 20%, 40%, 60%, 80% of volume loaded conditions in regular waves and irregular waves. Due to the developed tanks with a swash bulkhead and a swash hole, a roll resonance frequency of the vessel is shifted and roll motion of the vessel with developed tanks is reduced comparing with IMO type C tanks in both regular waves and irregular waves.