In this paper, optimal parameters of interline power flow controller using particle swarm optimization is presented. Basic and inertia weight approach particle swarm optimization techniques are applied to optimal power flow control in an electrical power system. Based on the steady state model, the sizing of the controller in the network is formulated as an optimization problem to minimize the transmission line loss in the network. The power flow control constraints due to the use of interline power flow controller are included in optimal power flow problem in addition to the normal conventional constraints. Simulations results are obtained on standard IEEE 30-bus system to minimizing the transmission line losses and optimal parameters of the controller are derived. © 2010 IEEE.