In this study, experimental and nonlinear finite element analysis of strengthened steel-concrete composite beams is presented. A steel-concrete composite beam is produced by using a steel beam and concrete slab bonded each other with shear connectors. Strengthening is applied by Carbon Fiber Reinforced Polymers (CFRP) sheets to the lower flange of the steel beam. Three samples are prepared, one of them is considered as a reference sample, the other two are strengthened by CFRP sheets with different number of layers. In experimental study, steel-concrete composite beams were tested by 4-point bending test with cyclic loading. During the test, load, deflection, and strain values are measured. Then 3D finite element models of the steel-concrete composite beams are prepared using tetrahedral elements. Finite element analysis is performed by using ATENA nonlinear analysis program. The results of experiments and finite element analysis are compared. Results indicated that the strengthened steel-concrete composite beams have larger moment capacity, lower deflection that the steel-concrete composite beam sample. Some evaluations are made on especially in terms of strength, applicability, stiffness and energy consumption about the steel-concrete composite beams with CFRP. Experimental results are found similar to the results obtained by nonlinear finite element method.