A dual mode serial transmission apparatus and method for switching a mode thereof are provided. The dual mode serial transmission apparatus includes a first and second current sources, a first and second inverting circuits, a differential pair and a resistor string. The first inverting circuit receives a mode selecting signal or a first data transmission signal, the second inverting circuit receives the mode selecting signal or a second data transmission signal. First and second load terminals of the differential pair are respectively coupled to the first and second inverting circuits. A common terminal of the differential pair is coupled to the second current source. First and second differential input terminals receive the mode selecting signal or respectively receive the first and second data transmission signals. The resistor string is coupled in series between output terminals of the first and second inverting circuits.