An expansion adapter is used to communicate both PCI and AGP devices to the north bridge chip of a computer. The expansion adapter includes a first AGP bus control module communicable with the north bridge chip via a first AGP bus, and a second AGP bus control module in communication with the first AGP bus control module, communicable with an AGP device via a second AGP bus. The identifying codes of the first and second AGP bus control modules are set to show no AGP device function in order to allow the AGP device to communicate with the north bridge chip via the expansion adapter. The expansion adapter further includes a PCI bus control module in communication with the PCI device and the first AGP bus control module for controlling data transmission between the first AGP bus control module and the PCI device