The main objective of the work was to determine the antioxidant capacity of the flavonoids in red apples with and without skin, strawberries, tomatoes and fresh onions and thermally treated: humid heat (boiled and vapor), dry heat (oven) and high frequency (microwaves). The measurement was carried out in a fluorescence spectroscopy and the statistical analysis through the Variance and Test of Duncan. The results indicate that the antioxidant capacity of the flavonoids of red apples with and without skin, strawberries, tomatoes and onions were of 0.259, 0.267, 0.278, 0.165 and 0.223 Equivalent uM Trolox. These values diminished after the thermal treatment at a higher degree by dry heat at 0.128, 0.072, 0.077 and 0.048 Equivalent uM Trolox respectively and 0.146 Equivalent uM Trolox in the boiled onion. The flavonoids showed a higher antioxidant activity in fresh state, and their activity diminished under heat (cooking).