The purpose of this MSc thesis was to determine the content of trans fatty acid (TFA) in 102 samples of selected Slovenian supermarket foods. Samples were divided into 10 groups (instant soups, chocolate spreads, instant coffee drinks, chocolate products, salty snacks, ice creams, vegetable whipped creams, French fries, Wiener schnitzels, hamburgers). The fatty acid (FA) composition was determined according to AOAC 966.0 with method in situ transesterification and gas chromatography procedure. The content of TFA exceeded the maximum reference value (below 2 % of TFA) in 18 samples (salty snacks, instant soups and ice creams). Among all the analysed foods, samples in group salted snacks had the highest content of TFA. Most groups of food contained more trans-C18: 1 than trans-C18:2. Based on the results, we found that instant soups, chocolate products and salty snacks contained the highest amount of t10-C18:1 and t9-C18:1. Both of these FA can be found in partially hydrogenated vegetable oils. Group of hamburgers and ice creams on the other hand had the highest content of t11-C18:1. In group of French fries and Wiener schnitzels the highest content of t9,c12-C18:2 was found because this products are prepared by frying food in oil at high temperatures. Instant coffee drinks and vegetable whipped creams proved to be the most atherogenic and thrombogenic food group due to their high content of saturated fatty acids.