Composite steel-concrete beams are used widely in bridge and building construction as the main structural elements in flexure. These structures have a design life and this may be reduced if loads are increased or environmental degradation occurs. Strength of such structure members and thus replacement or retrofitting may need to be considered. One method of retrofitting (stiffening and strengthening) a composite steel-concrete beam is to externally post-tension the beam. The present study focuses on evaluating the effect of adding the external post-tension as a strengthening technique on the simple composite beams. The study was conducted using the finite element program "ANSYS". Nonlinear material models for the components of the composite beam were used in the finite element model. The outcomes got from finite element analysis were confirmed against available experimental results. A broad parametric study was conducted to explore the effect of tendons profiles on flexural behaviour. This covers: load deflection behaviour, converting the simple beam into partially continuous beam, verifying an external post-tensioned simple beam.