Abstract: In this paper, an investigation is carried out on how the hull form and arrangement influence the seakeeping and global hull girder loads of a trimaran. By means of a ship hull variation method, a series of derived trimarans with the same displacement and different length to breadth ratio(L/B)from 12 to 19 is generated for a 1 000 tons trimaran. The longitudinal motion and wave load of the ships with forward speed in regular wave are calculated with the WASIM code based on a time-domain three dimensional Rankine method. The statistics of the pitch and heave motion,sectional bending moment and shear force of the hull girder are analyzed combining the wave spectrum in sea state 4 to 6. It is observed that as the L/B increases from 12.27 to 19.16, the peak value of pitch and heave motion is decreased by nearly 60% and 35% respectively, but the shear force is tripled and the bending moment is increased by 3.5 times. Further direct calculation of the wave loads compared with British LR Rule showed that slender ship section shear significant value have already exceeded the Rules in sea state 6. These results show that the slenderness of the main hull has a contradiction impact on motion and wave load, namely the slender body, the smaller the motion response, the better seakeeping. But the body is slender, total longitudinal bending moment and shear force will increase sharply, more slenderness hull is likely to lead to increase the ship navigation safety risk on the high sea state conditions. There fore,it is important to consider motion behavior and global longitudinal strength together when determining the principal dimensions of trimaran main hulls.