On-line estimation of low-frequency electromechanical oscillations in an interconnected power system has attracted much attention as this provides vital information about the stability condition of the power system. In this paper, a measurement-based method for the estimation of the frequency, magnitude, and damping of an oscillation is proposed. The method can be applied to the phasor measurement data to extract further information about the power system. The proposed method is derived by applying modifications to the existing concept of second-order generalized integrator and adaptive notch filter. This paper details the mathematical derivation and stability properties of the proposed method and presents the simulation results to confirm the analytical derivations and the desirable performance of the proposed method.