This paper contains an adjustment of the stiffness and control algorithm of an ankle joint to maintain the posture of a humanoid biped walking robot using an IMU (Inertial measurement Unit) in single support phase. One of the difficulties in maintaining the balance of a robot in single support phase involves the sole of the robot, which is easy to separate from the ground. This phenomenon is caused by an external disturbance or when the sole lands on uneven terrain. A method of adjusting the compliance in the ankle joint is introduced to reduce the tendency of this type of occurrence. In order to maintain the posture of a robot with a compliant ankle joint, a posture controller composed of a body balancing controller and a vibration reduction controller is adopted. A walking experiment is implemented using a compliant ankle joint and the posture controller. The proposed method is shown to enable rapid walking as well as walking on uneven terrain.