Deals with basic research on underwater docking of flexible underwater structures using an active control system. The actively controlled underwater docking has such potentials on the installation or construction of underwater structures at deep water depth that the structures can be installed precisely on the desired point of seabed. For very large structure automated construction, it can be possible to assemble the onshore fabricated partitioned structures on site. In the paper, the H/sup /spl infin// controller is used for flexible structural control to avoid spillovers. Basin tests were executed using two types of neutrally buoyant flexible models with ultrasound ranging system and thrusters.