A novel single-mode large mode area polymer fiber with nano/micro cor e-cladding structure is presented in this paper.To address the high intermodal-dispersio n and narrow bandwidth of common multimode polymer fiber (POF) at the same time and find a po ssible way to meet the future demands of fiber to the home (FTTH),we built a single-mode pol ymer fiber model with nano/micro core.Based on the non-weakly-guidi ng approximation,we calculated the Maxwell equations and analyzed the mode field a nd optical waveguiding characteristics.Through discussing the influence of the nano/micro core diameter, core/cladding index difference and cladding diameter on the energy distribution and effective mode field diameter of the polymer fiber,we find that if the nano/micro core diamet er is small enough,the POF would still be single-mode even the core/cladding index differen ce is large,the determinant of mode filed is core/cladding index difference,not core (or claddi ng) index.When the wavelength is λ＝650nm,core diameter is D＝172μm,cladding diame ter is Dclad＝250μm, and core/cladding index difference is δn=0.128,single -mode POF reaches its best performance.The effective field diameter reaches 126.56μm and only about 10.66% energy will pr opagate in the nano/micro fiber core.