<p>A simplified method for optimizing it is proposed, because cross-deck structure is very important part in fishing catamaran structural design. This method consists of two parts-optimization of central longitudinal section and brackets of a cross-deck structure. First, dimension optimization of central longitudinal section is performed by genetic algorithm (GA). Here, new constraint conditions are reflected so that the wet deck of the cross-deck structure is not exposed to the water. Then, bracket optimization model of the cross-deck structure is made by optimization dimensions obtained by GA. To perform bracket optimization easily, we use a compartment model including bulkheads instead of global cross-deck structure in ANSYS. As a result, structural strength of model obtained by optimization is satisfied enough. In the future, it is believed that proposed method can be used to optimize the cross-deck structure of the catamaran in preliminary design stage.</p>