In this paper graph theory is used to identify coherent groups of generators and to locate PMUs for Inter-Area monitoring. Initially, the problem of identifying coherent groups is presented as a graphical problem. Then, three graph clustering methods are implemented to group coherent generators. Finally, when the electric network is separated in coherent regions, a placement of PMU based on centrality criteria is proposed. This can be used as a first stage in the implementation of a plan of monitoring in a large electric power system. Dynamic simulations and phasorial representation of simulations are done with the reduced order equivalent of the interconnected New England system (NETS) and New York power system (NYPS). The results show that graph theory can be applied to identify coherent groups and to locate PMUs to Inter-area monitoring.