The operation of a particular component in deteriorating condition will lead to a high machine downtime. This is due to the failure of component at unexpected time. As a result it will increase cost of maintenance and production lost. One of the solutions to this matter is to use Preventive Maintenance (PM). In industries, Preventive Maintenance (PM) is not a new practice to minimize the sudden breakdown of production machine. PM will be performed at predetermine intervals to provide a balance between failure cost and component utilization (aging). Therefore, the objective of this paper is to introduce the preventive maintenance strategy for determining an optimal replacement time for component that deteriorates over time. In this thesis we consider a particular type of machine (Gear Motor 7.5 KW.) from Holcim Bangladesh ltd. where machines are subject to maintenance. To maximize the benefit from operating the machine two replacement models are used. Among them, one model is used to determine an optimal replacement policy which tells us, when equipment reaches a particular age, whether or not it should be replaced or continue to be operated to minimize the total operating cost. Another model is used to determine the optimal interval between the preventive replacements to minimize the total cost and to operate the machine to the time which is determined by first model. We have determined to find the preventive replacement cost and also the maximum time at which we can use the machine without replacing it. Some time it is more economical to replace the machine rather than maintenance it. So it is most important to find out the age at which the replacement will be most economical to replace rather than maintenance.