This paper proposes a novel dynamic voltage instability expectation index which takes into account dynamic voltage stability characteristics, the long-term phenomena and probability state of power system contingency. This index is a meter to measure the general robust power system under critical cases which are identified by contingency analysis. The critical contingencies are precisely analyzed by steady-state power load flow in perturbations to figure out violation cases, the weakness of the power system. The probability states of each of violation cases are calculated through availability and unavailability values of devices. The degree of voltage instability is estimated newly by the smallest determinant of dynamic load flow Jacobian matrix in evolution time of power system simulation. The variables of dynamic devices such as under load tap changer of the transformer, excitation limiters of generator unit, affected on a long-term characteristic of the power system, are considered in dynamic load flow Jacobian matrix. Combining the voltage instability rate and the probability state, a novel index, namely voltage instability expectation in dynamic index, is proposed. This index reflects the mutual correlation between the voltage instability rate and probable contingency cases which result in either voltage violation, overload flow, or voltage instability. The new index is demonstrated in IEE 9-bus test system and actual Vietnam power system. It should be a new measure to make a decision of transmission system expansion planning.