A mobile robot based on novel line-walking mechanism is proposed for inspecting power transmission lines. The novel mechanism enables the centroid of the robot to concentrate on the hip joint to minimize the drive torque of the hip joint and keep the robot stable when only one leg is hung on line. After reviewing of the line-walking mechanism, power line inspection robot is described in detail. The pose adjustment analysis is carried out to make sure that the robot will keep in stable state when rolling on the power transmission line which forms a catenary curve. The obstacle-navigation cycle of the designed inspection robot is composed of a single-support phase and a double-support phase. The centroid of the robot will be adjusted to shift to the other leg to start a new single-support phase. The feasibility of this concept is then confirmed by performing experiments with a simulated line environment.