A motherboard assembly includes a motherboard and an expansion card. The motherboard includes an expansion slot, a storage device interface, a power connector, and a central processing unit (CPU). The expansion slot includes a protrusion, first signal pins connected to the CPU, first power pins connected to the power connector. The expansion card includes a circuit board. A storage unit, a display unit, a power circuit, and a serial advanced technology attachment (SATA) connector connected to the storage unit and the storage device interface of the motherboard are all arranged on the circuit board. A notch is defined in a bottom edge of the circuit board, to receive the protrusion. An edge connector is arranged on a bottom edge of the circuit board and includes second power pins connected to the power circuit, and second signal pins connected to the display unit.