SA508-3 steel is the key structural material extensively used in large components of third-generation nuclear power plants. For increasing the process efficiency of nuclear power plants, extremely thick cross-sectional heavy forgings are necessary for constructing large components for these plants. Owing to thick cross sections, the as-quenched microstructure of the center of heavy forgings is typically granular bainite, composed of bainitic ferrite and martensite (M) and retained austenite (A_R) (M-A) islands. An M-A island is an undesired microstructure that results in the SA508-3 steel having a poor low-temperature impact toughness after conventional tempering at 650^oC. However, it is difficult to tailor the as-quenched microstructure owing to the limited cooling rate during the quenching process. Therefore, the modification of the tempering process is a more feasible method to adjust the microstructure and improve the mechanical properties of heavy forgings. Herein, the decomposition of A_R within M-A islands during different transformation paths and its effect on the mechanical properties of SA508-3 steel have been investigated. The results show that clusters of ferrite and agglomerated M₃C carbides are formed during conventional tempering at 650^oC. These coarse M₃C carbides decorate the boundary of the cluster, reducing the impact toughness of the SA508-3 steel. Accordingly, the size and distribution of these M₃C carbides are tentatively modified by introducing pretreatments at different temperatures before conventional tempering at 650^oC. This modification is because, during pretreatments, A_R first decomposes into various transitional microstructures such as martensite, bainite, or pearlite, which further transform into clusters of ferrite and M₃C carbides during tempering at 650^oC. The results show that 400^oC is the optimal pretempering temperature to improve the impact toughness of SA508-3 steel. Microstructural observations reveal that during tempering at 400^oC, A_R completely decomposes into fine bainite comprising bainitic packets and high-density cementite particles. This provides additional nucleation sites for M₃C carbides inside the clusters during the subsequent tempering at 650°C, avoiding the formation of coarse M₃C carbides distributed along these cluster boundaries.