Slow coherency analysis is used to effectively determine the slowly coherent generator groups among weak connections for any given power system. A novel graphic theoretic method is proposed in this paper to simplify a large scale power system into a smaller network without losing optimal solutions. A multi-level recursive bisection graph partition method is employed to partition the reduced network into desired sub-networks with minimum generation-load imbalance. The relevant simplification rules and partitioning procedure are presented. The strategy is tested on the precascading August 14, 2003 scenario for the Eastern Interconnection. The simulation results show that the proposed islanding strategy is effective.