This research investigated the effects of adding rhenium and ruthenium with various contents to new modified nickel-based superalloys, which were prepared and cast using a vacuum arc melting process. Heat treatment processes were then carried out. The size, circularity and the area fraction of the gamma prime phase were characterized via scanning electron microscopy after solution treatment, precipitation aging and long-term heating processes. The addition of ruthenium seems to provide a stronger effect than adding rhenium by increasing the area fraction of the gamma prime phase after precipitation aging. Furthermore, rhenium and/or ruthenium additions maintain the precipitation of the gamma prime particles in a cubic shape. For phase stability analyses, the alloys were subsequently heated at 1173 K in intervals up to 1080 ks. The study showed that the coarsening rate of γ′ particles can be significantly retarded with the addition of rhenium and/or ruthenium, and topologically close-packed phase can occur with excessive rhenium addition. In addition, the highest addition of ruthenium content in this study strongly exhibited high gamma prime phase stability. Thus, increasing the content of ruthenium and decreasing the content of rhenium can inhibit the formation of the topologically close-packed phase.