An optical engine including an illumination system, a beam splitting element, a second dichroic unit, and a light valve is provided. The illumination system provides a visible beam and an invisible beam. The beam splitting element is disposed in transmission paths of the visible and invisible beams. After the visible and invisible beams leave the beam splitting element, the optical axes of the visible and invisible beams are separated from each other. The second dichroic unit is disposed in the transmission paths of the visible and invisible beams from the beam splitting element. The visible beam passes through the second dichroic unit, and the invisible beam is reflected by the second dichroic unit. The light valve is disposed in the transmission path of the visible beam from the second dichroic unit and converts the visible beam into an image beam. The image beam passes through the second dichroic unit.