System reliability assessment is among the main acts in the operation and maintenance of every industrial and service sector, which also applies to maritime transportation system. Safety is an important parameter for system reliability assessment and a top priority for maritime transportation sector due to its direct effects on the environment, human life, and goods transported. Because human factor plays a very critical role on improving safety onboard, reliability assessment becomes vital for researchers and decision makers. The human factor also has an important role in bunkering operations. Bunkering is a routine operation on a ship which has been the cause of several accidental oil spills in the past. To prevent environmental damages caused by oil spill, bunkering operation requires careful planning, utmost care and high performance of the ship's crew. In this regard, the prediction of human error probabilities offers an advantage to prevent environmental pollution at sea and also provides a high level of safety in maritime industry. Therefore, the aim of this study is to calculate human error probabilities in bunkering operation by using Success Likelihood Index Method (SLIM). This study can also be used as a decision-making tool by ship's crew for improving safety at sea and preventing ship-based oil spill through providing a strategic and effective approach in bunkering operations.