Ignoring the deformation effect, the original size principle is normally used to calculate the internal force and deformations of the bars in Mechanics of Materials, the calculation error is not discussed in the textbooks, however. In order to show the adaptability of the original size principle to calculate the internal force of plane statically determinate truss, a case of the two-bar statically determinate truss was also given. The calculation error of the internal force and the angle deformations are quantitatively studied to make a comparison between the actual internal force and that calculated by the original size principle. The research showed that the relative error of angle deformations and the internal forces of the two rods are nonlinear dependent on the external forces (the given force), tensile stiffness and the initial angle of the plane truss. If the strength and tensile stiffness of the bars can meet the requirements of engineering, the original size principle can be used to calculate the internal force of metallic bar truss under the conditions of small deformations.