In this research, we examined a simple method for maximum loads corresponding to long-term probability of exceedance Q=10^-8 due to sloshing of a spherical LNG tank and new type tank. Tank excitation test using large scale tank model was carried out and the characteristics of sloshing load was confirmed. Regarding to the sloshing load in the excitation direction, the tank load becomes large near the tank natural frequency. The mode transition to the swirling occurs when the excitation amplitude increases and the tank load responds nonlinearly against the excitation amplitude. We proposed a formula for the long-term maximum load considering with the characteristics of sloshing load by means of specifying the wave condition based on the severest wave condition method. It can be estimated accurately compared with the conventional long-term prediction method. By using a simplified formula, we have made it possible to reduce a large number of test conditions (amplitude and frequency) required in the conventional method to once.