With the increasing loads, changing generation mix, environmental limitations on the expansion of transmission system and increasing system complexity, power systems are being pushed to their stability limits. System instability behavior is often characterized by voltage instability, sometimes even leading to a blackout. With the installation of synchrophasor devices, real time voltage stability problem detection can be possible, which in turn can make fast and efficient online voltage stability control possible. This paper introduces a new software application ‘RT-VSMAC Tool’ for online voltage stability control of power systems. Simulation results using different IEEE test cases have been presented to show and validate the performance of this developed tool.