To reduce cold-start emissions that is a bottleneck of tightened regulation, a photocatalyst was suggested and studied. Conventional TWC can reduce more than 96% of harmful emissions after the activation, but under a cold-start condition it needs a lot of energy to activate. During the activation time (light-off time) harmful gases are exhausted without any purification and it makes the conventional TWC difficult to satisfy tightened regulation. A photocatalyst can be activated by light that has wavelengths lower than 380 nano-meters. The activation needs no time and loss of energy like a TWC.To apply the photocatalyst on a vehicle aftertreatment a photocatalyst system was suggested that uses non-thermal plasma as a light source of photocatalyst. The durability of a photocatalyst was also studied with hydrothermal aging method. The properties of aged catalyst were tested with surface area, phase transformation and conversion efficiency of HC.