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Reflection characteristics of metallic waveguide gratings with rectangular grooves are investigated with the effect of the Joule loss taken into account. For this purpose, the scattering matrix for each step discontinuity is first found by using the mode-matching method with an impedance boundary condition at the surface of the conductor. Then, the scattering matrix for the overall waveguide grating is obtained by generalized transfer matrix methods. Numerical results show that Joule loss occurs mainly at horizontal wall sections. In order to suppress the loss of a reflector grating, it is shown that it should have a deep grating depth with a small number of periods. This configuration coincides with that for a compact and wideband reflector grating, which can well support single-mode operation, as was pointed out in our previous work.