This paper proposes a development of “Low Light-off Three Way Catalyst” and results by evaluation with a high density thin wall substrate such as 1.0×10-4m / 9.3×105cells/m2 (4mil/600cpsi) and 5.1×10-5m / 1.4×106cells/m2 (2mil/900cpsi). For development of “Low Light-off Three Way Catalyst”, the material components were improved. For example, 1) three kinds of precious metals, 2) Pr compound to improve the oxygen storage performance at low temperature of cerium oxides, 3) Ba for thermal stabilizer to prevent from the sintering of precious metals. To optimize these three factors, the developed catalyst could lower 90K light-off performance and C773 NOx was improved 15% to compare with current Pt/Rh catalyst after 1373K, 24h thermal aging. And by the vehicle test, tail pipe emission could be improved about 23% and 10% for HC and NOx conversion performance respectively.And substrate structure is one of the factors to improve the catalyst performance. The developed catalyst materials were wash coated on the high cell density thin wall substrate to consider the effect of cell density for catalytic conversion performance. From these results, the catalytic performance was improved as increasing the cell density. To compare with 1.5×10-4m, 6.2×105cells/m2 (6mil/400cpsi) substrate and 2mil/900cpsi substrate, the tail pipe emission of HC was improved about 35% by vehicle test.