The authors propose an index to evaluate the damping effect of a power system on a dominant mode of electromechanical oscillation. The index is obtained from numbers of line flow fluctuations in which the dominant mode is strongly observed even in the normal state of the system. To extract the index from the fluctuations, a combined technique is developed which is based on FFT (fast Fourier transform) processing and principal component analysis. The technique is applied to the field data which have been acquired at a single tie-line of a power system in Japan. The regression analysis with respect to the index and the total generation of the system shows that the damping effect of the system becomes strong with an increase of the total generation. This property, which is called a droop characteristic, could be used to determine the steady-state stability region and to identify whether the system is in the normal or alert state. >