In the night of November 4, 2006, at around 22:10, the UCTE (union for the coordination of the transmission of electricity) power grid was affected by a serious incident. The paper provides a short description of the events that triggered the blackout and introduces the restoration of the power system. It analyses the root causes and other critical factors that resulted in the blackout. Furthermore, by considering the experiences and lessons, the security defense system in bulk power grid is constructed to prevent large scale blackout in the future.