In this paper, a new application of Superconducting Magnetic Energy Storage (SMES) for diagnosis of power systems is proposed. Basic experiments for diagnosis of power systems by use of SMES were carried out by use of small generators, artificial transmission lines and a small SMES. The SMES gives small power disturbances to the power system without affecting oparating conditions of the power system. The small power oscillations in the power system due to continuous pbwer disturbances generated by SMES are observed. The relations between the power disturbances and the power change of SMES are discussed. Natural frequencies of the power system are obtained by investigating the oscillations due to the sinusoidal power change of SMES. The relations between the natural frequencies and the steady state stability of the power system are discussed, too. The possibility of estimation of the steady state power system stability by monitoring the natural frequencies of the operated power system by use of SMES can be shown experimentally.