The effect of Hf content on the microstructures after heat treatment and stress rupture properties of DD6 single crystal superalloy was investigated. The results show that with increasing of Hf content in the range of current research, the residual γ/γ′ eutectic increases apparently, the size of γ′ particles in dendritic cores has no obvious change and that in interdendritic decreases after heat treatment. With the increase of Hf content, the stress rupture life increases at first and decreases afterwards, and the elongation displays fast increasing first, and slow rising afterwards at 1100 oC, 140 MPa. At last the relationship between the microstructures and stress rupture properties of DD6 alloy with different Hf contents was discussed