In order to investigate the fish behavior in the water tank, the three dimensional positioning system with two CCD cameras was designed. The positioning system was tested at the vertical circulation water channel with observational part of 1,500L$ imes$1,500W$ imes$500H mm and the circular water tank with 2,050ø sub(1)$ imes$1,850ø sub(2)$ imes$400H mm. The observational error of vertical direction was larger than that of horizontal direction, and the observational error became enlarged in all directions according to the increase of depth and distance from the visual axis. The maximum observational errors of horizontal and vertical directions at the circulation channel ranged from -1.7 cm to 1.8 cm (2.4%) and zero to 2.1 cm (4.2%), respectively. But the errors of horizontal and vertical directions at the circular tank ranged from -1.3 cm to 1.3 cm (1.3%) and zero to 1.3 cm (3.3%), respectively.