The dynamical effect of a free water surface in on open tank with a rectangular section is investigated considering the effect of the surface waves occurring in the tank. The results of the theoretical calculation show that the dynamical influences are large when : 1. the tank is located upwards. 2. the natural frequency of the ship is large. It may also be certain that the rolling motion caused by an exciting moment having a frequency coinciding with the natural frequency of the tank water becomes extinct in short time.