This chapter examines a method for predicting transient instability of power systems based on readability analysis of hybrid systems. The analysis is performed by computing reachable sets of unsafe sets in nonlinear hybrid automata that represent both continuous electromechanical dynamics of generators and discrete operations by relay devices. The unsafe sets are subsets of system states in which power systems show unacceptable operations such as stepping-out of generators. The transient instability of power systems can be estimated by investigating whether a system state exists in the reachable sets or not. The estimation is possible at any onset of accidental faults such as line and plant trips. The proposed method was applied to an analysis of single machine infinite bus (SMIB) system. The SMIB system consists of a synchronous machine, an infinite bus, and two parallel transmission lines. An infinite bus is a source of voltage constant in phase, magnitude, and frequency, and is not affected by the amount of current withdrawn from it. The effect of relay control to transient stabilization of power systems is estimated. © 2007 Elsevier Ltd All rights reserved.