Some test results of CNG (Compressed Natural Gas) fueled vehicle have already been reported by gas companies(1), but they say little about the qualification of CNG for automotive fuel.This paper reports on the combustion characteristics of CNG and establishes its qualification for alternative fuel and for clean fuel. CNG has many advantages when used in automotive engines. Since it comes as gas, it has higher octane number and allows a leaner burning than gasoline. Under designs specially created to take advantage of CNG, an unique engine system was achieved which would not have been realized with gasoline fuel.CNG has some demerits, however, such as decrease in power, poor component durability, and short driving range by one tankful. Thus its study should be continued to establish CNG as automotive fuel.