Power transmission systems are prone to several vulnerabilities which may result in deterioration of system stability and incapacities due to equipment contingencies. The causes leading to contingency and instability are ranging from natural disasters to malevolent acts as well as misoperations and equipment's hidden failures. In principle, most power systems are designed in an efficient manner in order to provide openness and accessibility for Transmission System Operator (TSO) personnel. As a result of this fact, the term power system vulnerability becomes directly related with the physical reach of either natural calamities or some terrorist acts. Thus, a measure for possible disruption events is needed in order to have a more generalized and comprehensive figure of vulnerability degree measurement required for analysis. In this study, a measure for power transmission line's terrorist attack vulnerability evaluation is presented using IEEE reliability test system with external constraints imposed on system data.