As position control alone is not sufficient for machine walking, force control is often suggested. The foot force control allows better force distribution among the legs and therefore reduces the chance of overloading the leg actuators. In this paper, a flexible gear system is designed between the motor and load, by which the force on the gear can be measured and the fuzzy position/force control can then be implemented. The flexible gear system also acts as a buffer to greatly reduce the contact impact magnitude and frequency, therefore the leg can interact with environment with a high speed. Equipped with the flexible gear system in a knee joint, a robot leg has been constructed and an intelligent walking strategy is applied for the leg to walk smoothly on an unknown terrain.