In recent years, most researchers have focused on methods which mimic natural processes in problem solving. These methods are most commonly termed "nature-inspired" methods. Ant colony optimization (ACO) is a new and encouraging group of these algorithms. The ant system (AS) is the first algorithm of ACO. In this study, an improved ACO method is used to solve hybrid flow shop (HFS) problems. The n-job and k-stage HFS problem is one of the general production scheduling problems. HFS problems are NP-hard when the objective is to minimize the makespan . This research deals with the criterion of makespan minimization for HFS scheduling problems. The operating parameters of AS have an important role on the quality of the solution. In order to achieve better results, a parameter optimization study is conducted in this paper. The improved ACO method is tested with benchmark problems. The test problems are the same as those used by Carlier and Neron (RAIRO-RO 34(1):1-25, 2000), Neron et al. (Omega 29(6):501-511, 2001), and Engin and Doyen (Future Gener Comput Syst 20(6):1083-1095, 2004). At the end of this study, there will be a comparison of the performance of the proposed method presented in this paper and the branch and bound (B&B) method presented by Neron et al. (Omega 29(6):501-511, 2001). The results show that the improved ACO method is an effective and efficient method for solving HFS problems.