A casing of an electronic device including a metallic housing, a first non-conductive spacer and a second 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 having a first gap and a second gap substantially located at two opposite ends of the metallic housing and being parallel with each other. The first non-conductive spacer is disposed the first gap, and the second non-conductive spacer is disposed in the second gap.