Abstract: Interface slip always exists in steel-concrete composite beams (hereinafter referred to as composite beams),it will also reduce composite effect and stiffness and increase deflection of composite beams.If the interface slip and deflection are to be calculated,closed solutions can be only obtained for simple cases,such as for simply supported beams with simple load cases.For a continuous beam it is difficult to obtain a closed solution.Moreover the closed solutions are often too long and too complicate for application.With the finite element method,it is efficient to calculate the interface slip and deflection and there is no restriction of loads and supports.The key step for the finite element method is establishing an element stiffness matrix,which is derived in this paper by using the principle of virtual work.In addition the interface slip and deflection of a simple supported composite beam is calculated by using the finite element program designed by the auther and the results are compared with those of the closed solutions.It shows a good agreement and the difference of both results is small and within 1%.Thus the element stiffness matrix derived in the paper is correct and can be used in the finite element program for solving a large number of practical engineering problems.