There are concerns about the impact that global warming will have on our environment, and which will inevitably result in expanding deserts and rising water levels. AUVs (Autonomous Underwater Vehicle) were considered and chosen, as the most suitable tool for conducting surveys concerning these global environmental problems. JAMSTEC has started to build a long range cruising AUV. The plan for its development is in several steps. As the first step an AUV, named "URASHIMA", was built in 1999, and sea trials have been held since 2000. URASHTMA dived to 3,518m depth in 2001. At the end of February 2005, the vehicle was able to cruise autonomously and continuously for 317km, beyond its target range of 300 km. Recently the AUV "URASHIMA" has begun to undertake cruises for scientific applications. These applications require precise maneuvering of the vehicle for detailed investigations. For high performance maneuvering of the vehicle, it is necessary to design a control system based on a mathematical model for the vehicle. To construct an accurate mathematical model, experiments on the maneuverability of the vehicle were done during sea trials. The experimental results were used in system identification for the mathematical model. This paper describes vehicle dynamics, presents experimental results of maneuverability obtained with the AUV "URASHIMA" during the sea trials, and presents calculated results after the system identified a mathematical model.