Preventive maintenance can be described as maintenance of equipment or systems before fault occurs. The main goal of maintenance is to avoid or mitigate the consequences of failure of equipment. It also improves reliability, decreases cost of replacement and system downtime. Preventive maintenance helps provide high level of availability of system function and obtains great return on investments of hardware and software. However, very frequent application of preventive maintenance is not economical though. In order to obtain optimum benefit, this maintenance should be done at proper times. This study aims at explaining the importance of preventive maintenance mathematically. The mean time to failure (MTTF) or the mean time between failures (MTBF) is taken as the reliability criteria. The reliability values obtained when preventive maintenance is applied and not applied are compared to the MTTF.