Whether it is steam powered or has an internal combustion engine as the main engine of propulsion, a ship without a boiler is unimaginable; somehow a boiler, which is indispensable on ships is needed. Boiler, one of the most important ship auxiliary engines, is used for several purposes that consist of heating of fuel tanks, machinery equipment and accommodation. Many problems can arise in boilers which are designed to work with main engine exhaust gas so that the waste heat can be used in order to match expectations of energy efficiency requirements. When ship systems are analyzed, it is understood that many systems are related to each other. If there is a problem with the boiler many systems will be affected and many problems will be encountered. Therefore, this study provides a fuzzy DEMATEL (Decision Making Trial and Evaluation Laboratory) method to assess most common critical operational faults in marine boilers. The DEMATEL method gives chances to identify and analyses the important faults of marine boiler with respect to cause effect relationship diagram. Likewise, fuzzy sets get through with the uncertainty in decision making and experts comments on DEMATEL. When the results are examined, it is understood that "boiler takes excessive time to reach pressure" is important in terms of the reasons. When the effects are examined, it is seen that the most important effect is "leakage from pipes or valves". The acquired results of the research will contribute to ship boilers operational safety and prevention of dangerous machinery effects.