A characteristic impedance of an electric transmission line is measured by way of extraction. In the method, a first probe and a second probe are provided, wherein the first probe and the second probe are separable and independently operable probes. A first characteristic impedance of a first circuit where a first terminal of the first probe and a first terminal of the second probe are directly interconnected to each other is first measured. Then a second characteristic impedance of a second circuit where the first terminal of the first probe and the first terminal of the second probe are connected to opposite terminals of the electric transmission line, respectively, is measured. The characteristic impedance of the electric transmission line can then be obtained according to the first characteristic impedance and the second characteristic impedance.