Oat is one of the grains used in the world, and consuming its right amounts could be considered as a part of a gluten-free diet for people suffering from celiac disease. However, this grain has a hull that is not digested by the human digestive system, and hence need to be dehulled. In this research, a laboratory impact oat dehuller was designed, fabricated and evaluated. The dehulling experiments of Hashemi oat variety were performed at 5 levels of rotor speed (1000, 1250, 1500, 1750, and 2000 rpm) and 3 levels of moisture content (6, 9, and 12% w.b.) with three replications. The effect of these variables on groat percentage (GP), dehulling efficiency (DHE), broken groats (%B), unbroken groat yield (%GY), and mill yield (%MY) was studied. The results revealed that with increase in the rotational speed of the rotor at all grain moistures, the %GY first increased and then decreased. The DHE indicated an ascending trend with increasing the rotor speed and grain moisture. The %B increased with increase in the rotational speed and decreased with increase in the grain moisture. The grains that were more resistant to dehulling showed also more resistance to breakage. At each grain moisture content, the rotational speed that led to the highest %GY was considered as the best rotor speed. The %MY had a direct relationship with %GP and DHE, and it showed an ascending trend with increasing the rotor speed and a descending trend with increasing the grain moisture. The best rotor speed and moisture content for dehulling the oat grains were determined to be 1750 rpm and 12% w.b., respectively.