This paper proposes a voltage stability index to predict voltage collapse in power system. The index is based on maximum loading capability of the bus combined with a thevenin equivalent method for the aggregated representation. The technique is applied to transmission test system. Using thevenin equivalent calculated by fast load flow or PMU data, the index can be used for online voltage analysis. Comparison of the index with techniques used in previous literatures are presented. New England 39 bus and IEEE 118 bus test system are used to analyse the effectiveness of the method.