Reducing the oxides of nitrogen (NOx) and particulate matter (PM) from Diesel engine exhaust gas has become very important problem in view of the protection of environment and the saying energy. But it is difficult to reduce both emissions significantly at the same time by engine modifications and operating condition control. Because, their characteristics have a trade off relation. Therefore many investigators are studying the aftertreatment technique of Diesel engine emissions such as a PM trap filter, EGR, etc. The aftertreatment technique is expected that engine operating condition limits will be loose and engine efficiency will be improved. A few years ago, Iwamoto showed that Zeolite catalyst could reduce the NOx with hydrocarbon reducing agents under oxygen rich condition*. After that, many researchers have been investigating Zeolite catalysts-. But it is difficult to put them to practical use, because of low NOx reduction ratio, poor durability to water vapor and particulate in exhaust gas. Further, the appropriate method of supplying with the reducing agent must be developed. Authors developed the platinum or copper ion exchanged Zeolite (ZSM-5) catalysts to cope with these problems. Further, the effects of agents, temperature, space velocity and long-term durability tests were performed on actual small size Diesel engine.From these tests, it was found that Pt/ZSM-5 catalyst had more than 50 % NOx reduction ratio with appropriate hydrocarbon reducing agent. And its performance had no significant deterioration after long-term durability test. It has become clear that Pt/ZSM-5 catalyst has a high potential for practical use.