A study has been made concerning changes in the fracture resistance of the grade R4 steel, largely used for manufacturing offshore mooring structural components, due to microstrutural variations. After the industrial manufacturing of chain links, specimens for tensile and CTOD tests were machined from the material. The first part of the experimental procedure was related to quenching and tempering heat treatments making use of different austenizing temperatures (860, 880, 900, 920 and 940(Degrees)C) and tempering at a given temperature (680(Degrees)C). On the basis of the CTOD at the maximum load presented by the material, the second experimental part was started, in which the optimal austenitizing temperature obtained in the first part (900(Degrees)C) was adopted and different tempering temperatures (650, 660, 670, 680, 690 and 700(Degrees)C) were selected. According to the CTOD testing carried out during the first experimental part, the fracture resistance of the material has increased when increasing the austenitizing temperature (860, 880 and 900(Degrees)C). However, the material`s toughness did not change significantly when quenching from the highest temperatures (920 and 940(Degrees)C). In the second experimental part, the CTOD values increased when increasing the tempering temperature, reaching a maximum value for 680(Degrees)C. Concerning the highest temperatures (690 and 700(Degrees) C) the fracture resistance of the material presented a slight reduction. The changes in the fracture resistance of the grade R4 steel, in both experimental parts, were associated with the microstrutural characteristics of the heat treatments.