New Ni-Co based superalloys (TMW alloys) for gas turbine disks have been recently developed. These new alloys are designed to have higher temperature capability than that of U720Li disk alloy by 50°C. The concept behind alloy design is to combine two kinds of γ-γ′ structure alloys, U720Li(Ni-Ni3Al) and Co-Ti(Co-Co3Ti) alloys. In this article, we describe the effect of solution heat-treatment conditions on the microstructures of TMW disk alloys (TMW1, 2, 4, and 2+4Co), such as the grain size and volume fraction of the primary γ′ phases and the hardness. Microstructural observation results showed that the average grain size and volume fraction of the primary γ′ phases are strongly influenced by the solution temperatures. For each TMW alloy, we identified the relationship among the solution temperature, volume fraction of the primary γ′ phases, and grain size that is necessary to achieve optimum properties. Vickers hardness testing results showed that the addition of a Co-Ti alloy has a positive effect on alloy strengthening at room and elevated temperatures. These results can help determine the optimum heat-treatment condition.