An expansion device for storage units is applicable to a data processing system such as a blade server, to allow the number of storage units connected with the expansion device to be adjusted or increased according to user requirements. The expansion device is provided with a first interface connected with the data processing system, and with at least one second interface connected with at least one storage unit. The expansion device further includes a transmitting unit and a switching module for allowing the expansion device to be connected with storage units using different transmitting standards.