Abstract: Slip occurs both at joist-slab interface and concrete-reinforcement interface of composite steel-concrete beams under negative bending moment. In this paper, a mechanical model is established to investigate the stiffness of composite beams considering two types of slip. In order to validate the model, a test of 3 full-scale specimen beams was conducted. The results show that slip always exists for composite beams under negative bending even with complete shear interaction between the steel and concrete components. The slip effect makes additional curvature of the beam and reduces the section stiffness by 10% to 20% in comparison with that given by transformed section method. Hence, the slip effect cannot be neglected in design. Formulae are also proposed to calculate the deflection of cantilever beams. The results can serve as a basis for further study on stiffness of continuous composite steel-concrete beams.