The interaction of superconductivity and magnetism is studied in iron based superconductor using the Hamiltonian consisting of the itinerant electrons, localized electrons moment, and s-f interaction. Using Greens function technique and equation of motion method, we have obtained an expressions for superconducting order parameters (△(T), △(0)) and critical temperature T_C, which reduce to BCS result in the absence of magnetic interactions. The result of the calculations shows that superconductivity can coexist with magnetism in iron based superconductor below the critical temperature.