The fruits of common walnut ( Juglans regia L.), known also as Carpathian, English or Persian walnut, have been part of the human diet for thousands of years, recently being discovered that they are rich in omega-3 fats and contain higher amounts of antioxidants than most other foods. Among other benefits, they improve brain health, prevent heart disease and cancer, treats cough and stomach ache. The aim of this study was to analyse the chemical composition of the fruits at full maturity from eight walnut cultivars, grown in a trial, at University of Craiova - Fruit Growing Research and Extension Station (S.C.D.P.) Vâlcea, Romania. Walnut varieties with different geographic origins and different agro-biological characteristics were used. The fruits of four Romanian cultivars (‘Jupâneşti’, ‘Sarmis’, ‘Unival’ and ‘Valcor’), three French cultivars (‘Ferjean’, ‘Fernor’ and ‘Franquette’) and an American one (‘Vina’) were analysed. For all varieties, the content of proteins, polyphenols, flavonoids, tannins, carotenoids, water and ash was determined. The walnut cultivars were grown in the same environmental conditions and the same cultural practices were applied, variability in chemical composition of the fruits was observed and depended on their genetic background, agro-biological characteristics and environmental conditions.