Sloshing in the liquefied natural gas (LNG) carrier occur due to the large dimension of the tank and the characteristics of the liquefied cargo. The description of nonlinear fluid flow and random tank motion in irregular waves are required to predict the sloshing phenomena. A numerical code adopting SOLA-VOF scheme has been developed to analyse sloshing flow. For the ship motion analysis 3D panel method is applied and the results are compared with those of ABS class. Furthermore, to evaluate sloshing load in the directional waves, this study used ISSC directional wave spectrum with cosine square directional spreading function. Ship motion time histories are used as the input data of sloshing calculation. We performed sloshing analysis for No.2 tank of 138K LNG carrier with fully loaded condition. Higher sloshing responses are predicted on head sea where the sloshing peak pressures are inversely proportional to the zero crossing wave periods than the other seas in high filling and severe sea conditions. The calculated maximum peak pressures considering the cushioning effects of corrugated membrane in head sea can be used as the external forces for the structural analysis.