The fossil fuels usage in the world has been increasing continuously due to the energy demand, which leads to a decline in fossil fuel reserve. The increase in the cost of fossil fuels and the environmental pollution lead a search for an alternative energy. The energy should be renewable, harmless and non-polluting. Used cooking oil methyl ester is derived through transesterification process by using Used Cooking Oil (UCO) and methanol in the presence of Potassium hydroxide (KOH) catalyst. Emission characteristics have been investigated the used cooking oil and their blends with diesel. B20K and B100K reduced the carbon monoxide (CO) by 4.5% and 32.27%, hydrocarbon (HC) emission by 28.26% and 45.65%, particulate matter by 3% and 19.13% and smoke emission by 7.25% and 28.27%, respectively compared to diesel at full load. On the other hand, B20K and B100K increased the NO<inf>x</inf> emission by 7.3% and 18%, respectively compared to diesel at the rated output. The emissions of carbon monoxide, hydrocarbon, smoke density and particulate matters were reduced and there was a slight increase in oxides of nitrogen with the used cooking oil methyl ester (UCOME), as a renewable harmless and non-polluting substitute for diesel engines. © 2018 Elsevier B.V., All rights reserved.