A server for or accommodating a plurality of first expansion cards. The server includes a housing, a half-width motherboard, a connector and a dual-sided riser card. The half-width motherboard is disposed in the housing. The connector is disposed at a side edge of the half-width motherboard for connecting with one of the first expansion cards. The dual-sided riser card is electrically connected with the half-width motherboard for connecting with another two of the first expansion cards, the dual-sided riser card being substantially perpendicularly disposed at the half-width motherboard, wherein the projection of one of the first expansion cards connected with the dual-sided riser card and the projection of the first expansion card connected with the connector are overlapped at the housing.