There occurs heavy fish tail and width shortage around the head and tail ends of the slab resulted from unsteady-state deformation during slab edging with flat cylindrical roll in roughing stands of hot strip mill. Full three-dimensional rigid-plastic finite element method (3D RPFEM) was used for numerical simulation of this rolling process. The calculated slab shape, separating force and rolling torque are consistent with those of experimental data found in the literature. The calculation precision is improved as compared with that of the theoretical analysis given in the literature. It implies that there is an obvious advantage for full 3D model and the technique to deal with singular point at the corner of the roll entry is valid.