A refractive index profile design, which can enable us to obtain a flat-top fundamental mode field around the center, is presented. The theoretical method to analyze the four-layer large flattened mode (LFM) fibers is given. The properties of the fiber, including the fundamental and higher order modal fields, effective area, and bending loss, are discussed. The reasons forming the different modal fields are explained. By discussion, the larger effective area of the LFM fiber compared with conventional step-index fiber is proven and the feasibility to suppress the higher order modes via bending is given.