In this paper, a simple genetic algorithm (GA) is presented for optimizing the structural systems with discrete design variable. First, the origin, basic process of GA was discussed. In order to improve the convergence of GA, considering the difficulty in choosing the probability of crossover and mutation, an adaptive GA, in which the probability of crossover and mutation were varied depending on the fitness values of the solutions, was introduced. Finally, the availability of the adaptive GA was shown by an example.