Mobile Harbor (MH) recently proposed by KAIST is a novel maritime cargo transfer system that can go out to a container ship anchored in the deep sea and handle containers directly at sea with the aid of the MH crane [1, 2].Since this system operates under the open sea condition, the MH crane should be newly designed to support inertia load and wind force as well as its self-weight.Wave-induced motions of MH such as rolling, pitching, and heaving generates a significant amount of inertia load, which is ignored in the design of conventional cargo cranes installed on the stable ground.Wind force is also critical to consider a higher level of wind velocity in the open sea.In addition to structural rigidity, mass minimization is important in the design because it generally reduces overturning moment and therefore enhances ship stability.In this paper, considering all the aforementioned loading condition, we derived the sensitivities of design-dependent loads (i.e.self-weight, inertia load, and wind force) with respect to design variables, and then applied topology optimization to the stage of conceptual design for the MH crane.We systematically obtained a design candidate through computational framework.