This paper describes the variation of coupled power with wavelength in the etched fused asymmetric single-mode couplers by using variational technique. According to the fibler parametera and sectional dimensions we analyzed the wavelength response of our lightly fused coupler using effective parallel waveguides. The theoretical curve of the wavelength-flattened coupler is in good agreement with the experimental results. The analysis indicates that the wavelength response of the coupler is sensitive to the diameter ratio of two fibres and the coupling length. This also agrees with the experiment.