Theoretical estimation methods of diffraction and radiation forces acting upon floating structures partly composed of permeable walls are developed. Combining the technique proposed by B. Molin for the treatment of permeable walls with the conventional singularity distribution method for the hydrodynamical representation of body geometries, the present method can be applied to 3-dimensional bodies of arbitrary geometry with permeable walls. Example structures were tested in a water tank and the calculated diffraction/radiation forces are compared to the experimental data, which verify the validity of the present method. A parametric study is also conducted while systematically varying the permeability as the parameter. From this study, the effects of permeability on the hydrodynamic characteristics of permeable floating offshore structures are discussed.