Trans isomers are formed in oils when they are exposed to high temperatures like refining and deep frying. The current study was aimed to evaluate the effect of antioxidants extracted from agricultural by-products on the suppression of heat induced trans-isomers formed in frying oils (soyabean and mustard) at 180 °C for 25 hrs. Quercetin was the predominant compound found in apple pomace extract (APE) and oleorupin was highest in olive leaf extract (OLE) from HPLC analysis. Antioxidants extracted from these by-products were added to the frying oil at different concentrations (100 ppm, 200 ppm, 300 ppm) both individually and in the form of blend of APE/OLE. It was found that best inhibitory effect on the formation of trans fatty acids (TFA) in both the oils was shown by APE/OLE-300 ppm followed by OLE-300 ppm, APE-300 ppm, APE/OLE-200 ppm, OLE-200 ppm, TBHQ (tertiary‑butyl‑hydroquinone)-200 ppm, APE-200 ppm and TBHQ-100 ppm. Thus, the inhibitory effects were majorly dependent on their kinds and concentrations. Natural antioxidants at a higher concentration performed better than synthetic antioxidants.