In this research, a development of a new 6 d.o.f. underwater manipulator which is actuated by electric motors capable of carrying over 20kg payload and of various operation under the water has been studied. The 25kg manipulator for applying to midium-sized AUV or ROV has been designed small and light but to handle a heavy 20kg payload. The kinematics of the manipulator has been analyzed. The joint actuator for the manipulator is designed and builted as a new modular typed double oil jacket for waterproofness. Also, superior joint torque performance of the developed joint actuator has been varified through tests in the air and in the water. To improve the design and performance of the manipulator, a new designed manipulator is introduced.