The oscillation of a fluid caused by external force, called sloshing, occurs in moving vehicles containing liquid masses, such as trucks, railroad cars, aircraft, and liquid rockets. This sloshing effect could be a severe problem in vehicle stability and control. Therefore, development of efficient and easy method to reduce sloshing effect is positively necessary. In this study, optimization design technique for reduction of the sloshing using evolutionary method is suggested. Two evolutionary methods are employed, respectively, the artificial neural network (ANN) and genetic algorithm (GA). ANN is used for the analysis of sloshing and GA is adopted as optimization algorithm. The considered storage tank for fluid is a rectangular tank. The design variables are width and installation location of the baffle, and sloshing reduction coefficient by baffle is used as an object function in the optimization. As a result of this study, the optimal design for sloshing reduction is presented.