To specify a design parameter of an extruded ceramic honeycomb body constituting a catalytic converter to thereby improve the performance thereof.

An extruded ceramic honeycomb body having an entrance side, an exit side and a plurality of parallel open-ended passages formed by intercrossing cell walls crossing the honeycomb body from the entrance side to the exit side. The passage has a rectangular cross-section having at least a rectangular, aspect ratio of about 1.2:1, and an effective surfacial area coefficient Nμ/Dh of at least about 70 in relation to the honeycomb is regulated by the hydraulic pressure diameter Dh and isothermic Nusselt number Nμ of the rectangular passage.