In developing countries, efforts to increase the efficiency of bus transportation, which is the most common type of urban public transport systems, have been increasing in recent years. Many studies addressing the performance of bus systems focus on approaches to optimizing user and operator benefits. In this study, a bi-level simulation/ optimization model is developed to optimize service frequencies in urban bus networks. At the upper level, a meta-heuristic Harmony Search (HS) optimization technique based model is proposed for solving an objective function that represents operator and user costs. At the lower level, the transit assignment problem, which represents the distribution of demand over the transit network, is solved. In the pro...