An analysis method based on minimizing the axial force by the upper bound method was proposed to obtain the optimal attack angle of the roller in the flow forming process. And the method for minimizing axial force was verified effectively by FEM simulation. The axial force of the roller in the flow forming has an essential effect on producing material build-up and bulge ahead of the roller. The optimal attack angle should minimize the axial force of the roller and also avoid defects such as build-up or bulge effectively, improving the manufacturing accuracy and the surface quality of flow formed product. The possible defects like the material buildup and the spring back or the elastic recovery could be numerically simulated by the FEM. It was shown by the elastic-plastic FEM simulations that the attack angle resulting in the least magnitude of the axial force also resulted in the smallest amount of the material build-up. It is concluded that the simplified calculation of the attack angle is practically useful.