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Machines can be unavailable in most real-life manufacturing systems for many reasons. In this paper, we investigate the problem of integrating production scheduling and preventive maintenance in a flowshop composed of M different machines in series, aiming to minimize the total weighed system cost. We assume that for a multi-machine series manufacturing system, whenever one of the M machines is stopped to perform a preventive maintenance activity, the whole system must be stopped. This assumption leads to an increase of preventive maintenance opportunities for the other machines in the system. Then, an integrated model is presented based on the opportunistic maintenance policy while taking into account the economic dependence of different machines. The model aims to search for an optimal combination of production and preventive maintenance planning. To facilitate the construction of the model, a judgment factor and a reason factor are introduced to describe the working state of the system and to explain the reasons for system downtime. The numerical example optimized with Genetic Algorithm on Matlab indicates that this integrated model can reduce the total weighted system cost by nearly 11.72%.