In this experimental study, the effect of the usage of vegetable oils, as alternative fuels, on engine performance and exhaust emissions in a single cylinder diesel engine were investigated. No 2-D diesel fuel and nine different oils (raw sunflower oil, raw cottonseed oil, raw soybean oil, and sunflower methyl ester, cottonseed methyl ester, soybean methyl ester, obtained from the raw oils respectively and refined opium poppy oil, rapeseed and corn oil) were used. In order to determine emission and performance characteristics, the engine was tested with full load-varied speed and constant speed-varied load tests. Chemical and physical properties of the raw oils were improved by transesterification. Test results showed that, when using vegetable oils, the engine performance and NOχ are lower, and smoke density is higher than the engine performance in which No 2-D diesel fuel is used. The engine performance with the methyl ester fuels is higher than that with the raw oils and are close to diesel fuel performance.