Abstract: The application of genetic algorithm in optimization design of structures with discrete variables is presented with an example of optimization design for three-bar hyper static truss structures with discrete variables. The comparison between the result obtained by traditional method and that by genetic algorithm indicates that genetic algorithm is more effective for structural optimization with discrete variables. Genetic algorithm is also used to solve a ten-bar truss structure with discrete variables. The effect of population on convergence of the algorithm is analyzed for the three-and ten-bar truss structures. The result reveals that the rational selection of the population is closely related with the chrome length.