In the previous report, the authors reported the results of experimental study on the effective breadth of the face plate and the skin plate of a curved girder such as bilge parts of a tanker structure. They showed that the effective breadth of a curved plate is considerably small owing to the hang down deformation of the plate, and that it is necessary to attach many brackets to the face plate and longitudinals to the skin plates for recovering the effectiveness of each plate. In this report, two theoretical methods, energy method and finite element method, are introduced to calculate the influence of brackets and longitudinals on the effective breadth of a curved plate and compared with the test results which were reported in the previous report. Theoretical results show good coincidence with the test results. Design curves, from which the effective breadth of the face plate reinforced by brackets can be obtained easily, are given. In case of skin plates, it is showed by the results of calculation by finite element method that the stiffness of longitudinals has influence on the effectiveness breadth of skin plates.