One of the most comprehensive FACTS devices is Unified Power Flow Controller (UPFC). This paper describes a mathematical model of UPFC derived from two-voltage source representation, which will be referred as Power Injection Model (PIM). The UPFC injection model is incorporated in Newton-Raphson (NR) power flow algorithm to study the effect of UPFC parameters , r and Xse on active and reactive power flow, total real power losses and reactive power losses, number of iterations, power mismatch and voltage profiles at the buses in a power system network. The model represents a more robust and feasible alternative to others because it is able to take operational losses of UPFC into account. In this respect, the effect of UPFC parameters for two cases viz., 0% losses and 2% losses within the UPFC has also been investigated in detail. A programme in MATLAB was written and a Hale network was used as a case study.