Abstract: The fourth generation single crystal superalloy DD15 rods were prepared by screw selecting method in a directionally solidified furnace. The long term aging of the alloy after full heat treatment was performed at 980 ℃ for 400 h, 800 h, 1200 h, 1600 h and 2000 h, respectively. The microstructure evolution of the alloy after long term aging for different times was investigated. The results show that the alloy consists of the regular cubic γ′ phase and γ phase after full heat treatment. The size of γ′ phase increases, but still in cubic shape, and the space of γ matrix channel increases with the increase of long term aging time. No TCP phase can be observed after long term aging for 1600 h. A little needle shaped TCP phase were observed after long term aging for 2000 h. Higher additions of refractory alloying elements Re, W, Ta, Mo and Nb in the alloy can suppress γ′ phase growing up and Ru can make the alloy more resistant to the formation of TCP phases. Therefore, the alloy has very excellent microstructure stability.