In this work, computational fluid dynamics (CFD) simulations have been conducted on leaked LNG vapor diffusion in which the gas is discharged from the vent post of a LNG powered RORO ship. A numerical model is proposed based on an open source Computational Fluid Dynamics (CFD) tool called OpenFOAM® for numerical simulation of the turbulent gas diffusion. The numerical method is first validated against a benchmark case and an experimental case. Two turbulence models, k-ε and LES, are applied and the numerical results are compared and discussed. By using the proposed numerical model, a series of LES simulations have been conducted on the LNG vapor diffusion of the RORO ship under several wind speeds and air temperatures.