To reduce the failure rate and maintenance cost of production systems, an opportunistic maintenance model was proposed for a series production system which experiences stochastic degradation. A stationary gamma process was used to model the uncertain degradation for each unit of the system. By introducing integrated recovery factor of imperfective maintenance, a condition-based opportunistic maintenance policy was proposed. A formulation was developed for computing the expected value of the maintenance cost over a defined time horizon. The optimal inspection interval of the system could be found by minimizing the expected maintenance cost. Finally, a simulation experiment was conducted to show that the proposed model was feasible and valid.