Computer aided design of resonant iris filter in circular waveguide technology is described here using Mode Matching Method (MMM). This numerical technique is first used to analyze the key element of the filter namely circular to rectangular waveguide discontinuity or circular to circular waveguide discontinuity. The generalized S-matrix of these discontinuities that form the filter structure are then cascaded to obtain the performance of the filter. Using optimization techniques the analyzed filter dimension are altered to improve its performance. Two filters using rectangular and circular irises have been designed and presented here. The MMM analysis of key elements in the filter have been verified with measurements available in literature.