A transmission cable includes a transmission cable, a first circuit board having a first power layout area, a first ground layout area, and a first circuit trace, and a second circuit board having a second power layout area, a second ground layout area, and a second circuit trace. The transmission cable includes a power wire, a pair of signal wires, and a drain wire. When the first power layout area, the first circuit trace, the second power layout area, and the second circuit trace are connected via a resistor, respectively, the drain wire connected between the first circuit trace and the second circuit trace will act as another power wire. Thus, a voltage drop at both ends of the transmission cable can be reduced by increasing the power wires to improve signal transmission quality.