Abstract: Peg-holes with key are non-round peg-holes in industrial applications.Compared with round peg-holes,assembly processes for non-round peg-holes are complex and difficult.This paper analyzes geometric relations and force relations in assembly process.Calculative formulation of inserting forces is given.The formulation shows that a additional inserting force is applied.In order to reduce the inserting forces,this paper presents two methods: stiffness series and seting auxiliary freedom.Theoretic analyses and texts show that the methods above are efficient for reducing inserting forces.