Side-to-side tanks are the most economical for an arrangement of cargo tanks of middle-sized tankers with double-hull structures. Liquid sloshing is a matter of some concern for these tankers. Experimental study is carried out on sloshing loads on the middle-sized tankers with double hull structures. A ship motion simulator is developed for the sloshing experiment, which gives a tank model 4-degrees of freedom motion (quasi-3D motion), rolling, heaving, swaying and pitching. Tank models are of two kinds, one is two-dimensional and the other is three-dimensional. Basic models of 2D and 3D are rectangular shape with no inner structural members and are modified to investigate effects of bilge hoppers and shoulder tanks, effects of center-line girders, and that of horizontal girders on a transverse bulkhead. Sloshing impact pressures are measured at inner surfaces of the models and on the inner structural members. Model tests are carried out for regular rolling, swaying motion and irregular rolling motion. In each case, fill depth and period of motions are changed. Period of each measurement of the model tests are 300 seconds. Peak values of impact pressures are detected, processed statistically and evaluated by 1/10 highest mean values. As a result, basic aspects of sloshing loads are shown through the experiments: effects of fill rates on resonance frequencies, effects of inner structural members on sloshing impact pressures, and effects of swash bulkhead to reduce sloshing. Measured sloshing loads in irregular waves by model tests are compared with the estimates computed by a linear probabilistic theory. They agree comparatively well, and the discrepancy between them seems mainly due to the nonlinearity of sloshing phenomena. The relation between impact pressure and loads still remains a subject to be studied.