This paper presents the results of an investigation aimed at correlating the gradient of microstructures with the mechanisms of fracture and with J-toughness in the heat-affected zone (HAZ) of submerged arc welds on a high strength structural steel. The microstructure of the weld metal is composed of refined acicular ferrite while the microstructure of the coarse grained heat-affected zone (CGHAZ) is basically formed by coarse bainite with a small proportion of martensite and primary ferrite in the grain boundary of the large prior austenite grains. A significant decrease in toughness is observed with an increasing incursion of the fatigue crack front into the CGHAZ. The degradation in toughness is definitely brought about by the CGHAZs of the welds, the size and distribution of these zones being relevant factors in the weld performance.