Fall is a risky event in the elderly people’s daily life, it often cause serious injury both in physiology and psychology. An inertial and magnetic sensor based system is designed for fall detection in real time. This paper presents the design and test of this system. The system consists of three attitude measurement units fixed on the human legs and waist. Each unit contains orthogonally mounted triads of accelerometers, magnetometers and gyros. With an integrated microcontroller for attitude calculating and flash for data storage, the size of the unit is 32mm×23mm×12mm. An extended Kalman filter based on quaternions is designed for attitude measurement. The digital angle output rate is 100Hz. Human lower limb kinematic model is presented. The position of the gravity center is tracking by the inertial and magnetic sensing system.