The purification performances of catalytic converters (Pt, Rh, Pd, Pd/Rh, Pt/Pd, Pt/Rh/Pd, Pt̲Pd and Pd̲Cu) were investigated to select suitable one for newly developed liquified natural gas(LNG) fueled vehicle. Two types of the construction of the catalytic converter, single-bed type and dual-bed type using two different catalysts in series, were used. A natural gas engine that has been modified the 3 cylinder gasoline spark ignition engine of 0.55 liter displacement equipped with newly developed manifold injectors was used for the investigation. The conversion performance of the exhaust gas at different catalyst temperatures was discussed with main emphases on methane conversion. The durability of the catalyst and the optimum loading of Pd catalyst were also investigated. To understand the suitable operating condition for the developed LNG engine, the effect of air excess ratio and engine load were examined. Formaldehyde emission characteristic with various engine conditions was also investigated.