Abstract: A Nickel based single crystal superalloy was prepared by screw selecting method in vacuum directionally solidified furnace. The microstructure and its stability were investigated by SEM and TEM after the alloy experienced standard heat treatment and then was aged for 200, 400, 600, 800 h at 1 070 ℃. The results show that the regular and cube γ′ phases were obtained after fully heat treatment of the alloy. The γ′ coarsening and the γ matrix channel broadening with time were observed in the long term aging at 1 070 ℃. The full γ′ rafting formed after long term aging for 800 h. No TCP phase precipited after long term aging for 200 h. Needle shaped TCP phase precipitated on dendritic crystals in the alloy after long term aging for 400 h. With aging time extension, the precipitated TCP phase increased and grew up from dendritic crystal region to interdendritic region. The TCP phase was mainly composed of Re, W, Mo and Co and so on. TCP phase was identified as σ phase, which precipitated and grew along fixed direction, 001 σ//112γ , (11-0)σ//(22-0)γ。