An on-board diagnostic (OBD) adapter includes an input connector, a first output connector and a second output connector. The input connector is electrically connected to the first and second output connectors through a circuit board. The input connector is inserted into a socket of the OBD to capture vehicle driving information from the OBD and transmit the vehicle driving information to the first and second output connectors. The first output connector is formed in a specification the same as the socket of the OBD. The second output connector is formed in a specification different from the socket of the OBD. The adapter provides connection with the OBD with the same or different connector to improve usability.