Abstract: The narrow-band band-stop filters are often used in microwave systems to suppress the nonlinear harmonic output of high-power transmitters and the parasitic passband of band-pass filters. The difficulty of the design is to eliminate the parasitic stopband at three times the center frequency f0. This paper effectively solved the problem of parasitic stopband at 3f0 and largely extended the upper passband by means of loading a lumped capacity at the open end of the parallel-coupled line. According to this method, a narrow band-stop filter with a center frequency at 5. 3 GHz has been presented. The test result shows that this filter has a good performance at upper passband.