European hazelnut (Corylus avellana L.) is one of the most widely used nuts because of its nutritional and health-beneficial properties. This study was carried out to determine some biochemical kernel constituents including fatty acid composition and protein content in 25 different hazelnut cultivars collected from three regions in the north of Iran. The total saturated fatty acid content and the total unsaturated fatty acid content were significantly different. Also, the amounts of both main saturated and unsaturated fatty acids were found to be significantly different. Oleic and linoleic acids were the main unsaturated fatty acids existing in the studied cultivars. The highest (78.02%) and the lowest (72.89%) content of oleic acid were determined in the cultivars ‘Vartashenʼ and ‘Atrakʼ, respectively. Also ‘Ganjehʼ contained the highest (10.91 %) and ‘Bootkhanekiʼ contained the lowest (9.48%) levels of linoleic acid, respectively. The major saturated fatty acids were palmitic acid (ranging from 5.73% in ‘Alipourʼ to 6.87 % in ‘Atababaʼ) and stearic acid (ranging from 2.14% in ‘Atrakʼ to 3.62 % in ‘Nemsaʼ). In addition, protein content ranged from 14.75% (Kolaparak) to 17.74% (Gerde-Eshkevar). The results obtained from the studied cultivars can be important for establishing new orchards and also in breeding programs. Manuscript profile