A method of changing a light transmittance of a head-mounted display device including a display and an optical lens and a head-mounted display device. The method includes identifying an application displayed on the display of the head-mounted display device in front of a portion of the optical lens; identifying a first light transmittance based on a type of the application or a type of a content of the application; changing, by a processor, a light transmittance of the optical lens to the first light transmittance; and displaying the content of the application on the display while the light transmittance of the optical lens is maintained in the first light transmittance.