Voltage instability has recently become a challenging problem for many power systems. It is one phenomenon that could happen in power system networks (PSNs) due to its overloaded condition. The result would be the occurrence of voltage collapse which leads to total blackout of the whole system. Five systems variable based VSIs namely Fast Voltage Stability Index, Line Stability Index, Line Stability Factor, Voltage Collapse Point Indicators, and New Stability Index are taken in to consideration to screen out the most vulnerable or weakest regions. In addition, this thesis work also compares and identifies the quickest index in response to voltage instability and also gives ranks of the whole network based on their index values. After maximum load increment, power flow program is conducted using Newton Raphson for index calculation. Consequently, the performance of these indices is tested on IEEE 14 and 30 bus test system using MATLAB to validate their practicability in the existing PSNs. The simulation results show that NSI has the best performance among all other indices in predicting the voltage instability of the network. All lines are ranked based on this index under contingency conditions. The contingency case reveals that line 2, 3 and 9 are critical lines for IEEE 14 bus test system. As well as line 5, 12, 36, and 37 are the weakest lines for IEEE 30 bus test system. Their index values are 0.9876, 0.9281, 0.9887, 0.9987, 0.9190, 0.9391 and 0.9267 for both test systems. Furthermore, the D index dealt separately due to its distinct physical nature of the equation and the result shows that it senses the load quickly at the base case, but the index grows very slowly after repeated load increment.