Abstract: In order to shape a floating body all at once by fused deposition modeling 3 D printer, an optimization algorithm to balance floating bodies with interior support-free structures was proposed. First, a recursive subdivision method is used to create the self-supporting lattice structure inside the input model on basics of its preset floating posture, and then the lattice structure and the input model are assembled to an initial floating body model. Next, for the initial model, a balance optimization equation with lattice cell volume as variables is constructed and solved. The resulting model is generated by changing the volume of each lattice cell in the initial model, based on the solution result of the optimization equation. Experimental results show that the resulting models generated by the algorithm can be printed directly by a fused deposition modeling 3 D printer. The printed objects can not only float in the liquid according to their predefined floating postures, but also have high pressure resistance. In addition, thanks to the recursive subdivision method, which takes advantages from the perspective of saving printing materials, more lattice cells are generated inside input models.