Abstract: A more accurate moment method solution for longitudinal slot in a rectangular waveguide is presented, taking into account both distribution of the transverse electrical filed across the slot aperture and the finite wall thickness. Resonant length of the slot is calculated. It is shown that the calculated results have quite high accuracy, compared with both measured results and simulated results using software. Convergence performance of the method is discussed. It is found that the method will converge with up to about 20 basis functions and 2020 waveguide modes. Influence of the transverse electrical field distribution on calculating resonant slot length is also analyzed. The results show that the influence is great in cases including semi-height waveguide, large slot offset, wide slot width and thin wall thickness.