A casing of an electronic device including a metallic housing and a first non-conductive spacer is provided. The metallic housing has an inner surface and an outer surface opposite to the inner surface, and the outer surface has a back side and lateral sides connecting with the back side. The inner surface is substantially a recessed structure. The metallic housing has a first gap communicating the inner surface and the outer surface, and the metallic housing further includes a second gap and at least one connecting terminal. The first non-conductive spacer is selectively disposed in the first gap of the metallic housing, and extends from a first side of the lateral sides of the metallic housing to the back side of the metallic housing.