Abstract: The resonant metal iris in rectangular waveguide,which has finite thickness,is investigated with mode-matching method.The generalized scattering matrix of the iris is derived.Based on the theoretical analysis,a three-cavity band-pass filter composed with resonant irises and a step impedance transformer are simulated respectively as practical examples.The results agree very well with that of HFSS and the calculation time is only one third of HFSS,which verified the validity of this technique.