Structural characterization of edible oils after thermal treatment was made by (FTIR) spectroscopy. Olive, corn and sunflower oil samples were thermally treated at three consecutive cycles. Each cycle consisted of heating at the boiling point for 8 h, followed by cooling at room temperature for 16 h. At the end of each cycle the characteristics of oils were studied by FTIR spectroscopy and standard methods. Study of FTIR spectra of the treated oils revealed that the heating of oils caused significant changes in the intensities of their bands and produced shifts in the position of specific bands such as 3008 cm−1 and 1745 cm−1. The level of oil oxidation during different heating period was followed by the absorbance ratio at 3530/3472 cm−1. Additionally, the absorbance ratio 2853/3008 cm−1 was used as a measure of degree of unsaturation. The thermal heating of oils produced an initial decrease in the specific absorbance ratios which could be attributed to the reduction of unsaturated fatty acid content due to oxidation process. The results in this study demonstrated that FTIR could be used for assessment of the state of oils by using specific absorbance ratios as indicators.