In the multi-stage former, a manufacture of hexagonal fitting nut was generated in a defective products about 70∼80% in the industry field. Products generated in defects manufactured to be a machining about 60%. Additional process increased a product cost and decreased a product rate. Therefore, it is important to predict and design a preform reducing defective products in the early stage of process design. So in the study Defects for manufacturing hexagonal fitting nut verified a cause through the finite element simulation. To reduce a defective generation. a preform designed and a designed preform verified through the finite element simulation. In conclusion, a generated defects when a hexagonal fitting nut manufactured should reduce if a round dimension of preform reduced and a part of opposition angle distributed in plenty a volume.