Stress rupture tests at 975℃/240MPa and microhardness measurements at room temperature were conducted in high W cast superalloy K416 with various Al contents. The results showed that the stress rupture life and microhardness decreased gradually with increasing the content of Al, the stress rupture life of specimens with medium and high levels of Al content were lower than the values in standard. Microstructural characterization indicated that little M₆C carbides and α-W phase were oberved when the Al content was 5.60% (mass fraction, the same below). The area fraction of M₆C carbides and α-W phase increased from 0.08% to 2.1% with increasing the Al content to 6.20%. Meanwhile, the area fraction of γ/γ′ eutectics increased from 12.6% to 21.8%. The increase of Al content showed little effect on the sizes and morphologies of grains, dendrites, MC carbides and γ′ phase. The precipitation of brittle M₆C carbides with high amounts and the depletion of solid solution strengthening element (W) were responsible for the diminished stress rupture property. In addition, the stress rupture mechanisms of K416 alloy with various Al contents were also discussed.