Due to limited land and dense population, warm weather, and short distance between working location and residence, it is convenient and popular in Taiwan to use motorcycle as transportation means, which also becomes a special local feature. According to the statistics on vehicles, the number of vehicles reached 12,547,973 by the end of 1991, among which 9,232, 889, i.e. 73.4 percent, was motorcycle. It was estimated that the percentages of air pollutants emitted by motorcycle in respective total vehicle emission in 1991 were as following: CO 37.7%, HC 60.8%, NOx 2.9%. From the above statistical figures, we can comprehend the seriousness of HC pollution from motorcycles.Since November 1, 1984, R.O.C. has applied a standard to control CO and HC concentrations in motorcycle emission. Beginning from July 1, 1991, the emission standard became more stringent, as lowering CO emission to 4.5 g/km and HC plus NOx to 3.0 g/km (test procedure is the same as ECE 15). By the end of March, 1992, 10 motorcycle models have met the standard and obtained the approval certification. The engines of four-stroke motorcycles are redesigned to enable them for secondary air injectiojn, and two-stroke motorcycles are installed with catalytic converter to meet the emission standard. From July 1, 1993, all newly produced motorcycles in Taiwan are required to meet the emission standard, and the model number is expected to decrease drastically then.In order to effectively decrease the pollution from in-use motorcycles, the Environmental Protection Administration is actively promoting the Motorcycle Inspection and Maintenance (I/M) System. In compliance with the Air Pollution Control Act, motorcycles are required to be inspected periodically. Motorcycle manufacturers are responsible for the periodical inspection and maintenance, so as to decrease the pollution from the 9 million in-use motorcycles.The electricity-powered motorcycles will be another choice to decrease pollution from motorcycles. The electricity-powered motorcycle is scheduled to come into the market in May, 1992, it will be helpful in the reduction of pollution from motorcycles.