A new stopband filter consisted of dielectric grating waveguide was proposed and its filter characteristics were carefully examined by an approach of combining the muhimode network theory with the rigorous mode-matching method. The Brillouin diagram for the dominant TM mode and the variations of the normalized center frequency and the band width as well as the maximum attenuation constant of the stopband with the geometry parameters were presented. By comparing with the traditional dielectric grating stopband filter, it has been found that the new filter structure not only has the advantage of lower conductor loss, but also is of larger stopband bandwidth and maximum attenuation in the band than those of the traditional ones. The numerical results presented here are of guiding significance for the design of new band-rejected filters.