This work presents a proposed ringing test for the 1.5 GHz transmission lines using Nichols charts of self-loop function. The transfer functions of these transmission lines are derived based on the superposition principle. The simulation results of the phase margin at unity gain of the self-loop function in the coaxial line are 80 degrees (over-damping region), 60 degrees (critical damping region), and 11 degrees (under-damping region). It is shown that Nichols chart of self-loop function is a useful tool for the test of ringing in the distributed systems.