ABSTRACT Liquefied natural gas (LNG) exhibits significant environmental advantages. However, the hazards of LNG-fuelled ships need to be investigated in their early design stages, particularly with regard to accidental fuel leakage. In this study, a hypothetical oil tanker model was used with a fuel gas supply system installed on the main deck; the objective was to establish a critical zone for natural gas dispersion. A probabilistic approach was applied to determine credible scenarios, and leak size, location, and direction were considered as parameters along with environmental parameters such as wind direction and speed. Computational fluid dynamics was used to present the critical zone in terms of the contours of the natural gas concentration. The simulation results indicated that the leak size and wind speed significantly influence the size of the critical zone.