There have been observed several voltage instability phenomena in electric power systems where receiving end voltages oscillate remarkably or get much lower than the nominal values. Those phenomena tend to occur in heavy loaded conditions and seem to be related to the multiple load flow solution problem for the following reasons. It has been confirmed analytically and by simulations that the multiple load flow solutions are likely to appear under the heavy-loaded conditions, and that the individual solutions of the solution pair have different features from each other, from the standpoint of stability, although they are close to each other. It is hard to solve the voltage instability problem at a stretch since dynamic factors get involved in this problem and thus a high dimentional and multi-parameter system has to be dealt with. It may be wise to analyze, first of all, static and/or semidynamic performances of this problem, which could be approached more or less systematically. Next, the relationship between the dynamic factors and voltage instability will be analyzed and evaluated. Following the way of thinking above, this paper proposes a sequence of criteria which are applied to the individual members of a solution pair of the load flow to see their difference in behavior. The relationship between the voltage instability and closely-located solution pair is analyzed from various viewpoints for power systems of different types and sizes.