Abstract With the FACTS controllers, the power flow in systems can be flexibly regulated. In this paper, based on the steady state model of FACTS devices, their controllability of power flow is analyzed and control ranges of power flow are analytically investigated. With the developed power flow program in this paper, the power flow control diagrams of FACTS devices can be exactly calculated- Examples demonstrate the power flow control range is not only dependent on the device type, but also the natural power flow distribution on the transmission line. This calculated results are also correspondingwith the analytical.