A downlight apparatus includes a light source module, a cup housing, and a rotation structure. The cup housing has a surface rim and cup body. The surface rim defines a light opening. The surface rim covers an installation hole of an installation cavity. The multiple LED modules are disposed in the container space for emitting light out of the downlight apparatus from the light opening. The rotation structure selectively defines a first exterior diameter and a second exterior diameter. The first exterior diameter being larger than the second exterior diameter and an installation diameter of the installation hole. The second exterior diameter is smaller than the installation diameter.