The stressful condition of power system and high transmission losses leads to voltage security and reliability problems of system operation. If the voltage stability is not assessed and the problems occurred are not mitigated properly, cascading events may occur and this may lead to voltage collapse or blackout events. Therefore, there is a need to assess the voltage stability in order to detect the voltage collapse point and mitigate it when it occurs. This paper aims to compare four existing voltage stability index performance towards reactive and real power load changes. The performances are analyzed theoretically, and using simulation on IEEE 30-bus test system. Total power loss is also observed for each case. The comparison shows that all indices are prone to either reactive or real load power changes only. None of the indices are sensitive towards both power loads changes. Meanwhile, the total power loss is mostly contributed by reactive power losses.