An isothermal aging study was carried out on direct aging GH4169G alloy forging disk for up to 10000 hours at 650°C. The evolution of microstructure and mechanical properties of the alloy were systematically studied via scanning electron microscope (SEM), transmission electron microscope (TEM) and physicochemical phase analysis. The results show that the size of main strengthening phases γ′′ and γ′ in GH4169G alloy direct aging disk increases gradually, while their quantity decreases with prolonging aging time. The total amount of γ′′ and γ′ is 14.1% at as-heat treated condition, which decreases by 3.1% at 10000h. The amount of δ phase increases gradually, and the content of δ phase reaches 5.4% at 10000h, which increases nearly twice. The α-Cr phase adjacent to the δ phase on the grain boundaries shows a parabolic increase in its content, and reaches 2.231% at 10000h. The yield strength, stress rupture life shows varying degrees of reductions as prolonging thermal aging time.