A head-mounted display device including a front element, a pair of extending portions, an adjustable fixing assembly, a top adjustable fixing assembly, a rear adjustable fixing assembly, and a rotatable adjusting mechanism is provided. The front element is adapted to be placed or be installed with a display device and is adapted to cover eyes of a user. The top adjustable fixing assembly is adapted to be in contact with a parietal bone of the user. The rear adjustable fixing assembly is adapted to be in contact with an occipital bone of the user. The adjustable fixing assembly and the top adjustable fixing assembly are adjustable according to head shapes of different users. The rotatable adjusting mechanism adjusts the rear adjustable fixing assembly so that the rear adjustable fixing assembly is adapted to head shapes of different users.