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Voltage stability margin is essential to be known in advance to avoid voltage collapse and system blackout. This paper presents a new technique for determining the voltage stability margin, so that a corrective decision is taken in the proper time. The proposed algorithm is based on recursive least error square to measure the Thevenin's impedance at the bus in question as the busload changes. The proposed technique utilizes the past measurements together with the recent local measurements, bus voltage and load current, to predict Thevenin's impedance. The voltage instability occurs at a point where the load impedance equals Thevenin's impedance. Prior to this point the voltage margin can be predicted using the proposed technique. The method is tested on the IEEE 30 bus system for a single bus-load change, short-term voltage stability, and when the buses loads change with the same rate, long-term voltage stability study. Unlike the other techniques the proposed algorithm has the ability to estimate digitally the Thevenin's impedance at the load bus in question.