An optical device includes a carrier, a light source, a die, a light guiding structure and a reflecting structure. The carrier has a surface. The light source is disposed on the surface and configured to emit a light beam. The die is disposed on the surface and configured to sense the light beam. The light guiding structure is disposed on the surface and configured to guide the light beam. The light guiding structure includes a light receiving surface facing the light source and a light exit surface. The reflecting structure is disposed over the die. The reflecting structure includes a light reflecting surface facing the light exit surface of the light guide structure and is configured to reflect the light beam exiting from the light exit surface to the die. The light reflecting surface and the light exit surface are separated from each other and define a vent hole.