A measure related to resilience and vulnerability can be useful in supporting decision-making process for maintenance and repair strategies. The existing measures have been focusing on large-scale road networks like highways between cities, although urban-scale networks are equally important. This study proposes a new methodology for evaluating link criticality of urban road network based on the concept of macroscopic fundamental diagram (MFD). Such operational measure is derived by comparing the MFD behaviour in normal condition and the behaviour in event condition, in which a link of a road network is disabled by an extreme event. A simulation study for testing the proposed measure is practiced on the road network of Gangnam district, Seoul, South Korea. The results show that the measure can help us observe the different patterns of network flow reduction by different traffic states. This work provides a new perspective of investigating link criticality of road networks.