Today's power systems become more prone to cyber-attacks due to the high integration of information technologies. In this paper, we demonstrate that the outages of some lines can be masked by injecting false data into a set of measurements. The success of the topology attack can be guaranteed by making that: 1) the injected false data obeys Kirchhoff current law and Kirchhoff voltage law to avoid being detected by the bad data detection program in the state estimation and 2) the residual in the line outage detection is increased such that the line outage cannot be detected by phasor measurement unit data. A bilevel optimization problem is set up to determine the optimal attack vector that can maximize the residual of the outaged line. The IEEE 39-bus and 118-bus systems are used to demonstrate the masking scheme.