Ship bunkering is the process of suppling fuels to ships for their own use and it is a vital marine activity. This study focuses on the methods of bunkering operations, which carries many risks, similar to a number of other shipboard activities. Moreover, this paper analyses the causes of oil spillages during the bunkering operations. Specific guidelines, International standards and Conventions should be applied during all stages of the fuel transfer process in order to be ensured that the operations are executed with safety and diligence. Throughout the bunkering process, human life and the environmental protection are the main priorities. All the bunkering processes are detailed described, from the pre-delivery of the fuel until the completion of the process. The potential threats of oil spill during oil transfer are identified, as well as actual accidental events which have caused oil spills are described in this paper and their root causes are analysed. The catastrophic environmental consequences of the oil spills are described, along with the clean-up methods. In conclusion, the researcher concludes that the human error or omissions are the main causes of oil-spillages and recommends actions to avoid them.