This paper considers on-condition maintenance of a continuously degrading information system, in which the inter-maintenance time and the maintenance time depend on the condition of the system at which maintenance is carried out. The durations of preventive maintenance activities are supposed to be exponentially distributed. Assuming deterioration of the system follows a gamma process, models are established to maximize the system average availability when the effectiveness of the preventive maintenance activity becomes weaker and weaker. Optimal solutions on the condition of the system at which maintenance should be performed and the number of times of maintenance action to be carried out are obtained based on Monte-Carlo simulation. A case study is given to show the procedure of the maintenance model and simulation.