An augmented reality (AR) device is provided. The AR device includes a display module configured to output light of a virtual image, a waveguide configured to transmit the output light of the virtual image to an eye of a user and to pass external light therethrough, a gaze tracking sensor configured to obtain information on the eye of the user, the information including a pupil size, a memory storing instructions, and a processor configured to execute the instructions to control the display module to adjust a brightness of the light of the virtual image based on the pupil size.