With the development of phasor measurement units (PMU), real-time voltage stability monitoring techniques using phasor measurements have been widely discussed in the past decade. For various measurement-based techniques, the fundamental idea behind them is to identify the Thevenin equivalents (TE) of the outer system seen from the nodes/areas of interests, and then assess the voltage stability margin based on the equivalent circuits. Therefore, fast and accurate identification of the TE is crucial for such online monitoring applications. Though several identification methods have been proposed claiming to achieve good performance, they have not been explicitly compared with each other. This work presents an analogous comparative study of four different methods. After a brief introduction, the four methods are compared in the aspects of time complexity and measurement needed by using an algorithm analysis. Then they are tested on the measurements generated from time domain simulations of the Northeast Power Coordinating Council (NPCC) 140-bus system. Following the case study, a detailed performance analysis is given, and the results can serve as a general guidance on choosing the TE identification methods and the corresponding parameters. Keywords—adaptive (AD) method, coupled single-port circuit (CP), least squares (LS) method, phasor measurement unit (PMU), Tellegen’s theorem (TT), Thevenin equivalent identification, voltage stability assessment