We introduce a smartwatch assisted sensor fusion approach to robustly track 6-DOF hand movement in head mounted display (HMD) based augmented reality (AR) environment, which can be used for robust 3D object manipulation. Our method uses a wrist-worn smartwatch with HMD-mounted depth sensor to robustly track 3D position and orientation of user's hand. We introduce HMD-based augmented reality platform with smartwatch, and method to accurately calibrate orientation between smartwatch and HMD. We also implement natural 3D object manipulating system using 6-DOF hand tracker with hand grasping detection. Our proposed system is easy to use, and doesn't require any hand held devices.