A retaining assembly includes a cabinet (70), a first rail assembly located in the cabinet, a pair of metal plates (30) attached on the first rail assembly, a server (60), a second rail assembly attached to two sides of the server, two fastening plates (50) attached to the two sides of the server and two thumbscrews (90). A plurality of threaded holes (72) is defined in two sidewalls of the cabinet. Two first protrusions (14) are provided in the first rail assembly. Each metal plate has two positioning holes (32) and a fixing hole (34). The first protrusions engagably extend through the positioning holes respectively. Each fastening plate has an inserting hole (56) corresponding to the fixing hole of the metal plate. The thumbscrews are inserted through corresponding inserting holes of the metal plates to engage in corresponding fixing holes of the metal plates.