Real-time tracking of human body motion is an important technology in synthetic environments, robotics, and other human-computer interaction applications. This paper presents an ultra-miniaturized inertial measurement unit (IMU) named WB-3 for real-time attitude estimation of human limb segments. The WB-3 IMU is provided with a 32-bit microcontroller and 9-axis inertial sensors (miniaturized MEMS accelerometer, gyroscope and magnetometer). The quaternion-based Extended Kalman Filter (EKF) was implemented for the sensor fusion to retrieve the attitude of the human segment. An upper body motion capture system with 12 WB-3 IMUs was elaborated for tracking human movements in real time scenarios.