With the advantages of swimming with quiet, compliant, and continuous strokes, soft robot fish have significant potentials to be used in the study of marine life and ocean environment quietly without disturbing marine animals. Our project is to design and build a robot fish that is self-contained and capable of mimicking body kinematics of biological fish in nature. A new actuation mechanism is proposed in this project. It will efficiently transmit the motor power to the continuous oscillation of soft tail to allow the robot fish to maneuver in the water. The actuation's overall design is like a car engine, but instead, actuated by the motor and filled with granular elastic balls.