A head-up display (HUD) apparatus and a method of using the HUD apparatus are described. The HUD apparatus includes a first optical module configured to display a two dimensional (2D) image, a second optical module configured to display a three dimensional (3D) image and arranged at a predetermined angle relative to the first optical module, a separation module arranged on an optical path of the first and second optical modules to reflect a light of the first optical module and to project a light from the second optical module, and an optical system configured to output the light having passed the separation module.