A hard disk drive holder includes a bottom plate, a first side plate, and a second side plate. Gaps and a first bending line define a bendable portion and an unbendable portion on the bottom plate. The first side plate has first positioning portions. The second side plate has second positioning portions. The first side plate and the second side plate are respectively and integrally connected to opposite sides of the bottom plate. The first positioning portions and part of the second positioning portions are correspondingly disposed to install a first hard disk drive. The bendable portion has a plurality of third positioning portions, and the third positioning portions and part of the second positioning portions are correspondingly disposed to be adapted for installing at least one second hard disk drive. A size of the second hard disk drive is less than a size of the first hard disk drive.