Diverse theories have been established in voltage stability analysis since various aspects have been observed during voltage collapse phenomena. Through rigorous mathematical investigation, this paper shows that all the major methods used in static voltage stability analysis, i.e. Jacobian method, voltage sensitivity method, real and reactive power loss sensitivity method and energy function method-provide an identical result to show the uniqueness of static voltage stability analysis in theory. The tests for sample systems have shown that an identical result can be obtained from the various analysis methods.