Superconductivity in iron pnictides is studied by using a two-orbital Hubbard model in the large U limit. The Coulomb repulsion induces an orbital-dependent pairing between charge carriers. The pairing is found mainly from the scattering within the same Fermi pocket where usually one single orbital dominates. The inter-pocket pair scatterings determine the symmetry of the singlet superconductivity, which is an extended s-wave at small Hund’s coupling, and d-wave at large Hund’s coupling and large U. The former is consistent with recent experiments of ARPES and Andreev reflection spectroscopy. Spin triplet states only become important at large exchange interaction J.
