Abstract Mechanical damage to seeds occurring during harvesting and threshing, especially in naked cultivars, is one of the main factors decreasing the size and quality of yield. The objective of this study was to assess the susceptibility of naked oat cultivars to mechanical damage, considering the biometric parameters of seeds determined on the basis of computer image analysis and weight tests. The testing was carried out on eight cultivars harvested between 2008 and 2010 at 15% moisture and threshed at either 1.6 or 2.4 m·s−1 threshing drum speed. A 50% increase in the threshing speed caused an average 19% increase in the frequency and a 29% increase in the area of microdamage to seeds. There was a corresponding 4.1 mm2 change in the microdamage area when using the threshing speed of 1.6 m·s−1, and a 6.1 mm2 change when using the threshing speed of 2.4 m·s−1 was 53% and 68%, respectively, determined by a decreasing seed shape coefficient indicating seed elongation. The greatest resistance to mechanical damage was found on the Bullion cultivar, which was also characterized by the largest total projected area of seeds (7.14 mm2), as well as the greatest seed density (63.8 kg·hL−1) and thousand kernel weight (TKW) (28.2 g).