The wide band characteristic of fused fiber couplers is studied in this paper.Two fibers of stepping refraction index with different thickness at the coupled zone made by itching method are used to from an unsymmetrical arm. Thus a single mode directional fiber coupler is obtained with wide band and smaller size. Experimental results show that the coupled power can be flat distributed near a given wavelength and <100% splitting ratio by controlling the coupling length. A wide band coupler with different splitting ratio can be obtained by controlling the dianeter ratio fo two coupled arms.