An extension apparatus for a universal serial bus (USB) interface includes a transmitting device, a receiving device and an electrical signal network cable. The transmitting device includes the following elements: a first packet-processing unit to receive a first interface packet and generate an original data accordingly, a first buffering unit to temporarily store the original data, and a first data-converting unit to generate and output a network packet signal based on the original data. The receiving device includes the following elements: a second data-converting unit to receive the network packet signal and generate the original data accordingly, a second buffering unit to temporarily store the original data, and a second packet-processing unit to receive the original data and generate the first interface packet. The electrical signal network cable is electrically coupled between the transmitting device and the receiving device to transmit the network packet signal.