This paper investigates the ultimate strength of simply supported and continuous composite beams in combined bending and vertical shear by using the finite element analysis method. Three-dimensional finite element models have been developed to account for the geometric and material nonlinear characteristics of composite beams. The finite element models are verified by experimental results and employed to study the effects of the concrete slab and the degree of shear connection on the vertical shear strength of composite beams. The moment-shear interaction strength of composite beams is also investigated by varying the moment/shear ratio. The nonlinear finite element analyses show that the concrete slab and composite action significantly increase the ultimate flexural and vertical shear strengths of simply supported and continuous composite beams. Based on numerical results, design models are proposed for the vertical shear strength and moment-shear interaction of composite beams. The proposed design models consider the effects of the concrete slab, composite action, stud pullout failure and web shear buckling on the ultimate strengths of composite beams, and provide economical designs of composite beams in comparison with current design codes.