Wide Area Monitoring System (WAMS) using Phasor Measurement Unit (PMU) as the input devices is an advanced tool, which can help system operators monitor a power system condition, including voltage stability performance, in real time. The objectives of this thesis have two folds which are to determine the minimum number and location of the PMU to be installed given power network topology, and to propose the application of WAMS for monitoring voltage stability performance. The solution method of determining the minimum number and location of the PMU has been selected by comparing the effectiveness of the three methods proposed in the literatures, which are the Graph Theoretic Procedure, the Recursive N Method and the Generalized Integer Linear Programming. In this thesis, the Generalized Integer Linear Programming is chosen in regard to the observability performance. Then, the implementation of WAMS consisting of selecting suitable voltage stability indexes (VSI), employing linearly formulated state estimation technique, and designing graphic user interface (GUI) has been developed. The Synchrophasor-Based Real-Time Voltage Stability Index (SBRT VSI) and the PQ Voltage Stability Index (PQVSI), which suit well for on-line application, are used to identify the weak bus and the weak line rankings, respectively. The effectiveness of the developed WAMS has been successfully demonstrated using the simulated data of the IEEE 14-bus test system under four scenarios combining the effects of continued load increases at various load buses simultaneously, and short circuit faults. All the computed, on-line performance indexes including the critical point on the PV curve shown in the GUI have been verified against the actual values obtained later from the dynamic simulation of the system for the corresponding conditions.