Maintenance of complex systems is different from maintenance of individual units,because there are economic and failure dependence between units in the system.The optimal maintenance intervals of the units are not necessarily optimal in view of the entire system.For this reason,group preventive maintenance is presented for complex systems.First,maintenance cost and availability are analyzed.The optimal group preventive maintenance model is then proposed for complex systems.Subject to hazard,the model minimizes maintenance cost and maximizes availability.Finally,an air conditioning system is presented as an example in the case study,and the result shows effectiveness of the model.