The quantification of the total phenolic compounds of Pistacia atlantica showed that the different parts of the tree are rich in natural phenolic compounds. The antioxidant tests proved that the phenolic extracts have a strong antioxidant activity. The positive correlation between the Trolox equivalent antioxidant capacity (TEAC) and the amount of phenolic compounds confirms their contribution to the antioxidant activity. Among the various phenolic compounds isolated and characterized by spectroscopic methods, a new natural antioxidant 1 (methyl 5-(3,4-dihydroxyphenyl)-3-hydroxypenta-2,4-dienoate) derived from hispolone 2 has been isolated from the mushroom Inonotus hispidus growing on Pistacia atlantica. Hispolone 2 (6-(3,4-dihydroxyphenyl)-4-hydroxyhexa-3,5-dien-2-one) and hispidin 3 (6-(2-(3,4-dihydroxyphenyl)vinyl)-4-hydroxy-2H-pyran-2-one) have been also identified using spectroscopic methods, including 2D-NMR and EI-MS.