Abstract: The interfacial slip reduces the stiffness, increases deformation and influences performance of composite beams of steel and concrete. Since the loads on composite beams are large and the span-depth ratio is low, the shear deformation cannot be neglected. This paper presents the differential governing equations of composite beams based on minimum potential energy and variational method. Both shear deformation and interfacial slip are taken into account. The expressions for deflection and interfacial slip are obtained for beams subjected to concentrated and distributed loads. The results of the present method are in good agreement with other numerical solutions and experimental results. The expressions of additional bending moment due to interfacial ship for concentrated and distributed loads are presented, with which the additional deflection due to interfacial slip deformation can be estimated.