Composite steel-concrete beams are used widely in bridges and buildings 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 could happen. These changes may reduce the design life and strength of such members and thus replacement or retrofitting may need to be considered. The present study focuses on evaluating the effect of partial shear connection on strengthed composite beams with externally post-tension tendons. Using three dimensional F.E. modeling its able to simulate the overall flexural behaviour of composite beams which are strengthed with many shapes of tendons profiles. A fundamental point for the structural behaviour and design of composite beams is the level of connection and interaction between the steel section and the concrete slab. The use of partial connection provides the opportunity to achieve a better match of applied and resisting moment and some economy in the provision of connectors, taking into account the demonstrated advantages of externally post tension system like: Increase in ultimate moment capacity of structure, Enlarge the range of elastic behaviour before yielding for the structure with the introduction of internal stresses.