Abstract: A computer capable of automatic bandwidth configuration according to I/O expansion card (e.g., PCI-Express expansion card) type is provided. A motherboard of the computer includes an I/O expansion slot, a chipset, and a configuration setting circuit. When the I/O expansion slot supports different types of I/O expansion cards having multiple interface card slot combinations, a corresponding bandwidth configuration message is generated on the I/O expansion card. The bandwidth configuration message is used to indicate the type of the I/O expansion card that is being used and thereby control the configuration setting circuit to adjust the bandwidth configuration in the chipset.