A downlight apparatus has a tube body, a driver plate, a light source plate, a detachable attaching structure and a set of replaceable reflective cups. The tube body has an opening and a terminal portion opposite to the opening. The driver plate is disposed on an exterior surface of the terminal portion of the tube body. The light source plate is disposed on an interior surface of the terminal portion of the tube body. The detachable attaching structure is disposed in the opening of the tube body. One of the replaceable reflective cups is selectively detachably attached to the detachable attaching structure.