A bobbin used in a transformer is disclosed. The bobbin includes a main body, two side walls, two wire collecting bases and at least one extension wire collecting base. The main body has a tunnel. The two side walls are disposed on the two ends of the main body, respectively. The tunnel passes through the two ends of the main body and the two side walls. The two wire collecting bases are disposed on the two side walls respectively or on one of the two side walls. Each wire collecting base has at least one wire collecting part. The extension wire collecting base has at least one extension wire collecting part. The extension wire collecting base can couple with the wire collecting base.