Abstract: This paper discusses the Unified Power Flow Controller (UPFC) a member of the FACTS family. UPFC is able to control both transmitted real and reactive power independently, at the sending and receiving ends of the transmission line. Also, the paper discusses UPFC modelling within the context of optimal power flow (OPF) solutions. The UPFC model is very flexible; it allows the control of active and reactive powers and voltage magnitude simultaneously. It can also be set to control one or more of these parameters in any combination or to control none of them. Considerable progress has been achieved in UPFC modelling intended for conventional load flow studies but here more complex issue of UPFC modelling intended for OPF solution is addressed.