In this work, a numerical methodology for optimization of trusses with multiple objectives and constraints employing a modified firefly algorithm is presented. Both the weight minimization and the first natural frequency maximization of the truss are optimized at the same time so that the constraints on multiple frequencies are satisfied. The FA originally developed for dealing with a single objective function is modified in this work to handle multiple objectives. A development of the original FA relied upon Non-Dominated Sorting Genetic Algorithm II (NSGA-II) which is named as multi-objective FA (MFA) is presented. The MFA is first tested its reliability via benchmark mathematical functions. Then, it is applied to optimize multiple objectives of trusses under constraints for a ten-bar planar truss. In which, the design variable is continuous for the truss members’ cross-sectional area which is designed in a given space. A computer program done by Python software with version 3.7 on a laptop is utilized to investigate numerical examples. Moreover, results obtained for the truss are also compared with those given in the case of single objective optimization for validation. The proposed algorithm is fully promising for multi-objective optimization of other structures such as frames and plates.