Engineers have always thought about a solution that would extend the life span of their equipment. Preventive Maintenance (PM) is one of the most effective solutions. However, many considerations should be taken into account to perform it successfully. For example, factors such as probability of system operating normally (availability), probability of system being down for PM and probability of system failure. Also, selecting right time to do the PM works is a main concern. Besides, PM activities can improve reliability of certain equipment. This study uses the Feed Pump (FP) of a typical Hydrosulfurization (HDS) plant as an application. The main goal of this project is improving the effectiveness of PM. Therefore, the author have investigated in literature and found a suitable model. Each time one of the model input variables changes, while the remaining input rates are constant. The probabilities at normal conditions and steady state are calculated. In addition, reliability is determined using different model, which requires the failure rate values. The results indicate that the earlier PM is performed, the better.