When an offshore structure is damaged through a collision with a ship or an iceberg, certain influence from the fluid force acting on the colliding body is expected. However, reports on this subject are limited in number and the provided information is rather fragmentary. The characteristic of the phenomenon changes with the relative speed of collision. In other words, it depends on the relative duration time of collision. In general, the duration time is determined not only by the mechanical property of the collided structure, such as its stiffness and strength, but also the shape and the mass of the colliding body. To obtain general view of the phenomena for both when the behaviors of the structure are elastic and elastic-plastic, the authors studied the effects of the fluid force under collision through experiments and numerical analyses. It is shown that the degree of the effect of fluid force can be classified by parameters which are related to the relative duration time of the collision. Further, the degree of the effect of the fluid force is shown to be predicted using the proposed parameters.