A computer system includes a chassis, a motherboard secured in the chassis, and a mounting bracket. A first socket and a second socket is disposed on the motherboard. The mounting bracket is secured to the chassis and located above the motherboard. A first riser card is coupled to the first socket, and a second riser card is coupled to the second socket. The computer system further includes a positioning member attached to the second riser card. The positioning member has two opposite side plates. One of the side plates is secured to the second riser card, and the other side plate is secured to the mounting bracket and aligned with the first riser card in a same plane.