The increasing rate of the occurrences of several memorable events, such as 9/11 attack in 2001, hurricane Katrina in 2005, and tsunami in 2011, shows the necessity of the concepts related to resilience, risk, and vulnerability in terms of minimizing the negative impact at the stage of post-disaster. Particularly in the field of transportation and infrastructure engineering, various ideas and methods of measuring road network vulnerability have been proposed, since the beginning of the new millennium. This study firstly reviews the definitions of the conceptualized terms that are related to vulnerability of road network system to understand the exact meanings of the terms. The study also reviews the previously proposed vulnerability measure methods to find out the limitations that ought to be concerned in this study. The literature review is continued to understand the concept of Macroscopic Fundamental Diagram (MFD), which is a monitoring tool that can predict the capability of a road network system. Then, the basic idea of relating the MFD concept with vulnerability measure is proposed, and such idea is validated through a simulation study. The simulation results are analyzed by comparing the MFD behavior in normal situation with the ones in extreme event situations, in which one or more road links are disabled. The results and analyses of the simulation have brought such an encouragement to suggest a new type of vulnerability measure for urban road network system. The MFD-based Vulnerability Index (MVI) is proposed at the end of this study, and the index presents a fine result in measuring the vulnerability of an urban road network system. The results and analyses of the simulation performed in this research have brought some valuable topics for further researches as well.