A method based on oscillation energy for low frequency oscillation analysis is proposed. In this method， the damping of an element is estimated， and the generators with negative damping torque and the external periodic disturbances are located based on energy flow in power networks. At first， it is proven that the energy dissipation of a generator is consistent with its damping torque. Then， based on wide area measurements， a formula to compute oscillation energy flow in the power network is constructed without constructing energy functions. Because the energy flowing into a branch is composed of transient energy variation and energy dissipation or production， a practical method for energy flow computation is proposed to reduce the transient energy component and only obtain the energy dissipation or production. Because the total energy in the power system is consistent with the oscillation amplitude， the element dissipating energy has positive contribution to oscillation damping and has positive damping， while the elements producing energy are oscillation sources. The results obtained by using energy dissipation of a generator to estimate its damping torque coefficient are consistent with the conventional damping torque analysis. In addition， the energy flow can be used to locate the oscillation sources.