A head mounted device include a displayer, an eye-tracking module and a control module. The eye-tracking module is configured for tracking positions and movements of two pupils. The control module is communicated with the displayer and the eye-tracking module. The control module is configured to determine a target object located in front of the head mounted device according to the positions of the pupils. The control module is further configured to obtain a gap distance between the pupils and the target object. The control module is further to calculate a dimensional parameter of the target object according the gap distance and the movements of the pupils. The control module is further to display the dimensional parameter on the displayer.