It is proposed a novel wavelength filter with a narrow bandwidth suitable for WDM networks, which is based upon nondegenerate four-wave mixing(NDFWM) process in the active region of a wavelength selective coupler. By separating the desired wave-mixing product on the basis of the transverse spatial mode of the electric field, a bandwidth (full width at half maximum, FWHM) of subangstrom for the filter is obtained. The selected wavelength can be tuned in the entire semiconductor gain bandwidth (～70 nm). A theoretical analysis for NDFWM effect in the filter is given. The numerical simulation has been done. The influences of configure parameters on the output efficiency, 3 dB bandwidth (FWHM) and frequency shift are discussed.