The objective of this paper is to investigate the application of FACTS devices to increase the maximum loadability of the transmission lines which may be constrained by a transient stability limit. Hence, the on-line fuzzy control of the superconducting magnetic energy storage (SMES) and the static synchronous series compensator (SSSC) are suggested. The fuzzy rule-bases are defined and explained. The validity of the suggested control strategies are confirmed by simulation tests. The simulation results show that by the use of the proposed method, the line power transfer can be increased via the improvement of the transient stability limit. Finally, the effect of the control loop time delay on the performance of the controller is presented.