A server includes a casing, a motherboard, a power supply unit and a pair of expansion modules. The motherboard is disposed in the casing, and the motherboard includes two slots disposed in the casing. The power supply unit is disposed in the casing and under the motherboard. The expansion modules are disposed in the casing, and each of the expansion modules includes a connecting card and a plurality of expansion cards. When the connecting card is electrically coupled to one of the slots of the motherboard, the expansion modules are disposed at two opposite sides of the motherboard correspondingly.