Spintronics,which exploits electron spin in transport phenomena,has been a new branch in condensed matter physics in recent years.[1] Determining how to control and manipulate the electron spin is an important issue in spintronics,and modeling of spin filters is of great significance.Hence,many theoretical[2-7]and experimental[8] schemes have been proposed to design spin filters and to increase the efficiency of spin polarization.Graphene,a single atomic layer of carbon with low intrinsic spin-orbit couping and a long spin diffusion length,[9] has opened up a new area for spintronics.It was proposed to be used as an effective spin-filtering material when sandwiched between two layers of ferromagnetic materials.[10] Graphene nanoribbons (GNRs) with zigzag edges can be made to carry spin currents under exteal fields,[11] which may enable a new region to explore spintronics at nanoscale.A spin-effect transistor has been proposed by utilizing a graphene layer as the channel,where spin manipulation is achieved.[12] Zhang et al.investigated spin-dependent electron transport through a zigzag graphene nanoribbon (ZGNR) sample modulated by configurations of ferromagnetic strips and found that it could be applied as a spin filter.[13]Saffarzadeh et al.also proposed a spin filter device based on armchair graphene nanoribbons (AGRNs)recently,which perfectly passes one type of spin current through the system.