A microwave photonic filter (MPF) based on four-wave mixing (FWM) is proposed and experimentally demonstrated. Two single-frequency laser beams produce the four-wave-mixing effect in a highly nonlinear fiber and generate a multiwavelength optical signal output. The multiwavelength optical signal is used to generate the multitaps of the MPF. The wavelength spacing of the multiwavelength optical source is equal to the frequency difference of the two input laser beams. By changing the frequency difference of the two input laser beams, wavelength spacing of the multiwavelength optical source output can be continuously tuned in the range of 0.36 to 1.6 nm. Thus the center frequency of the filter can be continuously tuned within the range of 6.914 to 30.729 GHz.