Abstract: The Twiddler and head mounted display (HMD) are the most useful input and output devices for wearable computers, the tapping performance of Twiddler directly affects the controllability and usability of wearable computers. According to Fitts's law, the model of tapping performance in the Twiddler-HMD mode is established. Based on the model of tapping performance, the relationship between average tapping time and information quantity is researched. The result shows the icon is suitable for tapping operation of the wearable computer, when the ratio of the icon size to the width of graphical user interface (GUI) reaches 1/15.