This paper deals with the most basic and fundamental problems, the motion analysis of an unmanned submersible for underwater exploration. Some shape parameters derived from the analytical results for designing the vessel to be automatically controlled are also considered. The analysis is based upon the slender body theory for obtaining the linearized characteristics of the motion. The shape parameters related to the motion and control of the vessel are extracted, discussing optimum relationship in order to design the control system to be stable and controllable. Finally, some results of experiment are shown to verify the theoretical characteristics of the parameters to be valid.