Abstract: A cubical spline interpolation function is put forward to show the tendon profile of prestressed bars because the tendon profile can't be accurately expressed by the quadric parabola. With the cubical spline, the spatial location of tendon's inflexion and the way to calculate prestressed internal load are both given theoretically and the nonlinear distribution of prestressed internal load is revealed. In addition, two samples of prestressed RC frames of single floor and span are tested with a numerical simulation done by ANSYS. The results of experiments and numerical simulation are very near. Thus, a conclusion is drawn that the tendon profile shows a cubical spline interpolation curve and that the prestressed internal load is distributed nonlinearly.