In this paper, we present a hybrid body state estimator for the locomotion stability control of a bipedal robot with passive dynamic gaits. The hybrid system is based on the sensor data of a three-axis accelerometer and a digital compass which provides independent measurements of the bipedal locomotion and accurate sources of the disturbance information during passive dynamic walking and running. In addition, a fuzzy logic based algorithm is implemented in the system which performs feedback control. The body state estimator may play an important role not only in modeling and system stability but also in real-time control. Satisfactory results are obtained in a test platform that we create to evaluate bipedal locomotion with different gaits.