AbstractHazelnut (Corylus avellana L.) is an important shelled nut with its pleasant aroma. The refining process causes significant changes in the quality attributes of hazelnut oil. The aim of this study was to assess the aroma, aroma‐active, and phenolic compounds of the crude and refined hazelnut oils by using gas chromatography–mass spectrometry (GC–MS), GC–MS‐olfactometry (GC–MS‐O), and liquid chromatography–mass spectrometry (LC–MS/MS). The antioxidant capacities were also determined by DPPH and ABTS methods. Results showed that terpenes and aldehydes constituted a significant portion of the aroma profile. Refining process dramatically reduced the numbers (from 63 to 25) and amounts of aroma compounds (from 36,769 to 4461 μg/kg). Similarly, the numbers of aroma‐active compounds were reduced from 22 to 8 by the refining process. Their flavor dilution factors ranged from 2 to 1024 for crude oil and from 2 to 32 in the refined oil. As of the phenolics, five and two compounds were quantified in the crude and refined oil samples, respectively. In sum, the refining process had a considerable adverse effect on the aroma, aroma‐active, and phenolic constituents of the hazelnut oil; hence, the refining process has to be planned with minimal negative effect on its organoleptic properties.