This paper presents the experimental investigation result from a composite beam strengthened by shaped-steel underpinning (CBSSU) test. There were 3 specimens with one was common R.C beam to compare and others were strengthened by shaped-steel underpinning and jointed with short welding rebars. The main emphasis in undertaking this test was to show the effect of short welding rebars in the strengthening beams and collect more detailed data on the performance of CBSSU such as load capacities, load-deformation behaviors, slipping between shaped-steel and R.C beams, stresses distribution and failure mode. The test shows that the short welding rebars was effective to improve the bearing capacity and stiffness. With limited increase in height of the beam can substantially increase the ultimate flexural capacity and flexural rigidity. The cracks of the beam can also be controlled effectively.