The present invention provides to a novel grating-assisted directional coupler which is capable of appending a nonlinear thin film to a conventional linear grating-assisted directional coupler, to thereby significantly improve coupling efficiency and hence design a structure having optical multistable characteristics, which comprises; a thin film nonlinear optical waveguide with third-order nonlinearity, which having a first thickness; a first linear optical waveguide formed on the thin film nonlinear optical waveguide, which having a second thickness, wherein a linear refractive index of the first linear optical waveguide is more than that of the thin film nonlinear optical waveguide; an optical waveguide grating patterned in equi-distanced on top surface of the nonlinear optical waveguide; and a second optical waveguide formed at a portion which is displaced from top surface of the first linear optical waveguide by a predetermined distance.