On- line Monitoring of electric power system for controlled islanding addresses two main topics in power system monitoring. Firstly, two optimization models for solving the PMU placement problem with constraints of time and reliability are proposed; these models can be helpful for system planners in developing a roadmap of the process of selecting the location of the PMUs and establishing the guidelines of the long term process roadmap, Secondly, the application of PMUs in the controlled separation of power system is addressed.