A transmission circuit is provided. The transmission circuit includes a T-coil, a first resistance value generator, a second resistance value generator and a capacitance value generator. The first resistance value generator generates a first resistance value according to a first control signal. The second resistance value generator generates a second resistance value according to a second control signal. The capacitance value generator generates a capacitance value according to a third control signal. A gain spectrum of the transmission circuit is adjusted according to the first resistance value, the second resistance value and the capacitance value.