Currently, humans work in high-risk environ-ments such as earthwork sites with construction ma-chine. Such works are expected to be replaced by ro-bots, but autonomous technologies of current robots are not sophisticated enough to deploy to such tasks. Thus, it is reasonable to use robots that can assist hu-mans in cumbersome tasks and dangerous situation. These robots must have high repeatability, speed, power, and safety. As a preliminary study on human assistive robots, this paper designs and develops a powerful and backdrivable robot gripper. To provide powerful output suitable for construction works, we adopt an oil-hydraulic-driven actuator. To provide backdrivability for geometric and mechanical adapt-ability, we adopt magnetorheological fluids (MRFs), which can change its apparent viscosity, quickly, con-tinuously, and reversibly, based on the strength of the applied magnetic field, as the working fluids in the ac-tuation system. MRF largely affects the dynamic range of viscosity and response time, so we develop special type of MRF suitable for construction works. We then develop a small size vane type rotary actua-tor that consists of a passage in the vane and an elec-tromagnetic circuit to efficiently apply the magnetic field to MRF passing through the passage. The back-drivability can change based on the current applied to the coil and output torque can change based on the flow rate from the pump to the electro-hydrostatic ac-tuator. Finally, we develop a robot gripper (similar to the size of human hands) with two fingers (three inter-connected joints) actuated by one MRF actuator. From preliminary evaluation experiments, we con-firmed that the developed robot gripper could change backdrivability and output torque depending on the coil current and pump flow rate.