This paper described the sloshing simulation of the LNG (Liquid Natural Gas) tank due to the LNG ship's motion during operation at sea. The ship motions in irregular wave were obtained by 3D diffraction panel method in frequency domain. Coupled motions of surge, heave, and pitch due to the head sea of incoming wave were considered in the solving of longitudinal sloshing problem in certain range of wave frequency. The LNG sloshing on the Bilobe tank type was studied by using the Computational Fluid Dynamic technique with attention to obtain a maximum pressure that was occured on inner wall of the tank. Three cases of the LNG fi lling level including an empty (10%h), a half (50%h), and a full (90%h) conditions of tank height (h) were considered in order to investigate the free surface effect due to the LNG sloshing. The simulation results have shown that the maksimum pressure due to sloshing at inner wall have increased by 11.1%, 5.4%, and 11.5% while in the load conditions of full, a half, and an empty respectively. The maximum pressure that occurs did not exceed 6 percent based on the calculation of probability occurrence for all LNG fi lling level conditions.