A server includes a casing, a first disk drive assembly, a fan assembly, a host assembly, a first expansion card assembly, a second expansion card assembly, a power supply assembly, and a second disk drive assembly. The casing has a first to third space. The second space is located between the first and third space. The first disk drive assembly, the fan assembly, and the host assembly are respectively located in the first to third space. The first and second expansion card assembly are located in the third space and above the host assembly. The power supply assembly is located in the third space. The second expansion card assembly is located between the first expansion card assembly and the power supply assembly. The second disk drive assembly is located in the third space and located between the power supply assembly and the second expansion card assembly.