Transmission lines are prone to external excitations due to their low density and internal damping coefficient. They are vulnerable to some natural events such as wind and rain which may disrupt power transmission. Forced vibration due to external disturbances is one of the most frequent causes of failures in these lines. In this study, effects of tuned-massdampers (TMDs) on reduction of forced vibrations are investigated. A mathematical model for the transmission line is proposed by using mode summation technique combined with the dynamics of the absorbers. Best values for the location, stiffness and mass of the absorbers are found by using Particle Swarm Optimization algorithm (damping coefficients are obtained as zero). The objective of the algorithm is to minimize the line deflection to postpone the failure and the replacement of the lines. Results show that the designed TMDs are efficient in reduction of forced vibrations. Finally, the same procedure is implemented under resonance condition. The designed TMDs lead to a high reduction factor in the line deflection. Similarly, the proposed optimization algorithm can be used in other industrial applications of the flexible cables; under various external excitations.