A two mode image displaying apparatus including a light source, an image dividing unit, a displaying unit, and a control unit is provided. The light source is adapted to provide a light beam. The image dividing unit is adapted to be switched to include a three-dimensional (3D) mode area and a two-dimensional (2D) mode area. The light source is adapted to be switched to include a first area and a second area. The image dividing unit and the displaying unit are disposed on the transmission path of the light beam. The control unit executes a boundary brightness compensation by adjusting brightness of at least one of the boundary of a 3D image area and a 2D image area of the displaying unit and the boundary of the first and second areas of the light source. An adjustment method of image brightness is also provided.