In recent years, the need for large span members led to the spread of the prestressed concrete. But high self-weight and low ductility of prestressed concrete members are the most problems that face its application. One of the defects that face producing of lightweight aggregate concrete is weakness and brittleness of lightweight aggregate. Adding short discrete steel fibers play an important role in improving mechanical properties of concrete. This thesis focuses on study the behavior of post-tensioned lightweight concrete beams with different volume fractions of discrete steel fibers and a partial prestressing ratio. On the other hand, shows the numerical nonlinear model using a commercial Finite Element Analysis; the elements used in the model, and how discrete steel fibers was simulated in the model, the failure surface of the concrete, material properties, real constants, meshing, all assumptions, boundary conditions and solution are illustrated.