The natural frequencies of flexible fiberglass composite beams are controlled by activating optimal sets of shape memory alloy (NITINOL) wires which are embedded along the neutral axes of these beams. The underlying phenomena influencing the behavior of this class of composite structural members are presented. The individual contributions of the fiberglass-resin matrix, the NITINOL wires and the shape memory effect to the overall performance of the composite beam are determined at different operating temperatures and initial preloads of the wires. The modes of vibration of the fiberglass beams are measured with and without the NITINOL reinforcement at various operating conditions. With properly designed NITINOL reinforcement, it is shown that the beams can become stiffer and less susceptible to buckling. The modes of vibrations of the activated NITINOLreinforced composite beams can also be shifted to higher frequency bands relative to those of the unactivated or un-reinforced beams. Finite element model is developed to described the interaction between the NITINOL wires and the fiberglass-resin matrix. Close agreement is obtained between theoretical predictions and experimental results. With such tunable characteristics, the NITINOL-reinforced composite beams can be effective in attenuating the vibrations induced by various external disturbances.