AR is about to change how people observe and interact with the world. Smart wearable devices are widely used, their input interfaces, like button, rotating bezel, and inertial sensors are good supplementary for interaction. Further 6-DoF information of these wearables will provide richer interaction modalities. We present WatchAR, an interaction system of 6-DoF trackable smartwatch for mobile AR. Three demos demonstrate different interactions: Air Hit shows a way to acquire 2D target with single hand; Picker Input shows how to select an item from a list efficiently; Space Fighter demonstrates the potential of WatchAR for interacting with a game.