A computer server having a motherboard with a PCI riser card slot between a first upright bracket and a second upright bracket is disclosed a PCI riser card positioning structure, which includes a first locating hole formed in the top flange of the first upright bracket, a second locating hole formed in the top flange of the second upright bracket, and a auxiliary bracket bridged between the first upright bracket and the second upright bracket, the auxiliary bracket having a longitudinal panel and a PCI riser card mounted on the bottom side of the longitudinal panel, which having a mounting device and a locating plug respectively disposed at the two distal ends thereof and respectively fastened to the first locating hole and the second locating hole to guide the PCI riser card engaging into the PCI riser card slot for electrical connection.