A model is presented utilizing a Hamiltonian with equal spin singlet and triplet pairings based on quantum field theory and green function formalism, to show the correlation between the superconducting and spin density wave (SDW) order parameters. The model exhibits a distinct possibility of the coexistence of superconductivity and long-range magnetic phase, which are two usually incompatible cooperative phenomena. The work is motivated by the recent experimental evidences on high-T C superconductivity in the FeAs-based superconductors. The theoretical results are then applied to show the coexistence of superconductivity and spin density wave (SDW) in NaFe 1-x Co x As.