Abstract: Fractures during the uniaxial tensile test with round bar of two material models are analyzed by the elastic-plastic finite element method based on I₁ fracture criterion. The results indicate that the crack forms in the center of the minimal section at first. The crack unstably propagates into a circular crack the moment it forms, and then the propagation ceases. With the displacement load continuously increasing, the crack propagates step by step. The axial force reduces sharply when the crack forms and propagates unstably. The fracture strength I_b can be measured by using this characteristic of the axial force's change.