It is important to estimate accurately the maximum values of sloshing load in cargo tanks of membrane type LNG carriers in order that their reliability and safety may be maintained even in their severe service condition. Usually, model tests are carried out in order to estimate the sloshing load. But it is well known that measured impact pressure fairly scatters even in the case of harmonic oscillation. Then, we performed the statistical analysis by using both three parameter weibull distribution and Gumbel extreme distribution with upper limit in order to deal with the scattering of impact pressure. As the results, it is turned out that the application of Weibull distribution is useful to fit the experimental values and to estimate extreme values. Remarks in statistical analysis are also discussed in this paper.