Abstract: A fixing device is described. The fixing device is for fixing a power supply unit in a computer cover having a first side plate and a second side plate. The first side plate has a square orifice and a guide slot connecting to the square orifice. The fixing device comprises a rotary-tightening-up device. The rotary-tightening-up device, made of plastic, has a first side member and a second side member. The fixing device further comprises plastic hook connected to the rotary-tightening-up device. The plastic hook is guided by the guide slot, thereby rotating the rotary-tightening-up device to mount the power supply unit upon the second side plate of the computer cover.