With rapid advances in sensor, computer and communication networks, modern power systems have become complicated cyber-physical systems. The increasing use of information technologies in power systems has increased the risk of power systems to cyber-attacks. In order to evaluate the threat of attack on the power system, cyber attacks on smart grids aiming at misleading the control center with incorrect topology information are considered. In such topology attacks, an adversary intercepts network and meter data from the remote terminal units, modifies part of them, and forwards the modified data to the control center. A necessary and sufficient condition for an undetectable state-preserving topology attack and a more realistic and feasible attack strategy are analyzed and studied. The proposed attacks are tested with IEEE 9-bus and 14-bus systems. The simulation results show that the topology attack can have the destructive influence on the economic operation. This work is supported by National Natural Science Foundation of China (No. 61272486) and Research on New Theory and Key Technology of Cryptography in Cloud Computing Security Open Topic Foundation in State Key Laboratory of Information Security (No. 2014-02).