Providing a resiliency measure for power systems is one of the challenges towards its Dynamic Security Assessment. This paper introduces a resiliency measure, called Level-of-Resilience (LoR), determined by examining: (i) the Region-of- Stability-Reduction (RoSR), as the RoS evolves under attack and recovery actions as captured by a “modal-RoS”, (ii) the eventual Level-of-Performance-Reduction (LoPR), as measured by percentage of reduction of load served, and (iii) Recovery-Time (RT), which is the time system takes to detect and recover from an attack or a fault. We illustrate our measure by comparing resiliency level of two power systems under two different attack scenarios.