An image display apparatus and a method of adjusting brightness are provided. The image display apparatus includes a display panel unit which displays an image, a digital micromirror device (DMD) panel unit which reflects incident light using a plurality of DMDs to the display panel unit to provide a backlight of the display panel unit According to the present invention, a backlight of the display panel is provided by reflecting incident light onto the display panel using the plurality of DMDs so that the image display apparatus and a method of adjusting brightness thereof which improve a contrast ratio are provided.