In recent years, the use of bio-fibers to replace synthetic fibers in composites has gained popularity due to increasing environmental concerns and requirements for developing sustainable materials for engineering applications. The objective of this study is to minimize the weight and cost of a symmetric laminated composite beam with a specified lower bound constraint on its natural frequency. The optimal design of hybrid composite beams with different boundary conditions was considered. The optimization problem accounts for fiber type, fiber volume fractions, thickness, and fiber orientation angles of different layers as design variables. The optimum design of the hybrid carbon/flax/epoxy laminated beam for different values of frequency lower limits was computed and compared with those of the hybrid carbon/glass/epoxy, neat glass/epoxy, and neat flax/epoxy laminated beams. Results showed that hybridization between the carbon fiber and flax fiber results in the best designs with the advantages of a lightweight, low cost and higher fundamental frequencies.