Abstract： Proper maintenance schedule is required to improve the profitability and productivity of manufacturing systems. Thus, an opportunistic preventive maintenance model aiming at serial-parallel machining line consisting of much equipment was developed. Considering the specificity of this system, the concept of line balancing was proposed to get a better utilization of the equipment and balance the production capacity of each station. The opportunistic preventive maintenance model was based on reliability and made a target of the minimal maintenance cost. Proper threshold values as well as the maintenance scheme were obtained by Monte Carlo simulation of the model. An applied case shows that the model is practical and efficient to get a proper maintenance schedule for the series-parallel system.