Platinum (Pt), palladium (Pd) and rhodium (Rh) catalysts were evaluated on an engine dynamometer to examine the effects of air-fuel ratio (A/F), A/F amplitude and its frequency on the conversion efficiencies of NOx, THC and CO.Under steady-state conditions, Rh is more suitable for NOx conversion than Pt and Pd. The Pt and Pd catalysts give higher conversions in dynamic-states than in steady-states. With increasing A/F amplitude, A/F for the maximum NOx conversion is shifted to rich side, while A/F for the maximum THC and CO conversions are shifted to lean side. At the stoichiometric point, both Pt and Pd show the maximum conversions for particular frequency and their frequencies are shifted to higher values with increasing A/F amplitude.If the A/F amplitude and the frequency are selected to be optimum, Pt and Pd may be used as three-way catalysts (TWC). As a result of the durability test which was accomplished until 50, 000 km, Pd was promised as a main component of Rh-free TWC.