The recent discovery of iron-based superconductors 1-3 challenges the existing paradigm of high-temperature superconductivity. Owing to their unusual multi-orbital band structure 4,5 , magnetism 6 and electron correlation 7 , theories propose a unique sign-reversed s-wave pairing state, with the order parameter changing sign between the electron and hole Fermi pockets 8-14 . However, because of the complex Fermi surface topology and materials-related issues, the predicted sign reversal remains unconfirmed. Here we report a new phase-sensitive technique for probing unconventional pairing symmetry in the polycrystalline iron pnictides. Through the observation of both integer and half-integer flux-quantum transitions in composite niobium-iron pnictide loops, we provide the first phase-sensitive evidence of the sign change of the order parameter in NdFeAsO 0.88 F 0.12 , lending strong support for microscopic models predicting unconventional s-wave pairing symmetry 9-14 . These findings have important implications on the mechanism of iron pnictide superconductivity, and lay the groundwork for future studies of new physics arising from the exotic order in the FeAs-based superconductors. © 2010 Macmillan Publishers Limited. All rights reserved.