The properties of gasoline that is obtained from oil refinery, is improved with various additives prior to the release. Different additives produced by various firms are being sold in the market. These firms explain that these products improve combustion, provide fuel saving, clean the engine, reduce exhaust emissions and increase engine performance. In the present study, the gasoline additives produced by five different companies were used. It was considered that giving the names of the firms not right because of commercial reasons. So the additives were called as B, C, D, E and F has been called. These additives were blended with 95 octane gasoline at recommended proportions by companies. The effects of mixtures on a 4-cylinder 4-stroke spark ignition engine were experimentally investigated and the results were compared with pure gasoline. At the end of the experimental work, it was determined that the selected gasoline additives weren't enhanced improvement on the engine performance and exhaust emissions.