Traditional methods for power system oscillation analysis have some disadvantages;for example，high dependence on power system models and parameters，complicated computation and restricted to only offline analysis.Starting from the Dirac structure，this paper attempts to study a measurement-based methodology for power oscillation analysis，and to establish a theoretical foundation for this methodology.Firstly，the basic concept of the energy structure of a control system and its Dirac description form is introduced，and the energy structure of a general power system is given.Secondly，the idea of utilizing energy structures to analyze the influence of certain un-modeled dynamics to the system stability is introduced.Finally，the proposed methodology is adopted to locate oscillation sources in power systems with different precise extent，so that the validity，practicality and generality of this proposed methodology are verified.As the energy structure concept is applicable to all kinds of devices in power systems，this methodology is universal.Moreover，it is totally measurement-based，so it is computationally efficient and practicable.It is promising in many areas such as oscillation source location，oscillation type online identification and decentralized oscillation inhibition.