The use of cooking oils in the deep fat frying of food has increased significantly in the modern day living. During heating and frying the unsaturated fatty acids undergo isomerization and chemical reactions resulting in the formation of trans isomers and several other toxic compounds. During frying fatty acids are gradually depleted to form other products. The aim of this project is to study the changes in unsaturated fatty acid profiles in four different vegetable oils. Fatty acid profiles of soybean, rapseed, olive and corn oil samples that were heated at medium frying temperature 175°C in the presence of air were analyzed by gas chromatography after derivatization. Fatty acid profiles of the oils revealed that there is no significant formation of trans isomers at this temperature and time interval. The reduction percentages of unsaturated fatty acids are high in rapeseed oil. Furthermore, the results clearly show that the stability of fatty acids decrease with increasing number of double bonds in the molecule. Olive oil seems to be the stable oil at this frying temperature.