Through-thickness distributions of residual stresses were determined in an 80-mm thick ferritic steel weld using neutron diffraction and contour method. A two-wavelength (1.55 and 2.39 angstrom) combined method, which is based on minimized cross-section and attenuation, enables us to measure residual stresses in such a thick specimen using neutron diffraction. Moreover, the results were complemented by contour method destructively. Significant tensile longitudinal stresses (up to 660 MPa approaching the yield strength) occur along the weld centerline within 10 mm from the top surface, while compressive transverse stresses (about -100 MPa) were found near the weld mid-thickness. Grain size, hardness, and charpy impact energy were further examined to suggest a critical location of fracture associated with residual stresses.