The effects of Ru on the microstructural stability and stress-rupture property have been investigated for two experimental Ni-based single crystal superalloys with and without Ru (2.5 wt%) addition. The results of standard heat treatment and long-term thermal exposure at 1100 oC indicate that the intermediate γ′ precipitates in the dendrite cores of the Ru-free alloy do not change in morphology and only become coarsening after 2000 h thermal exposure. The Re partitioning ratio of γ/γ′ phases increases with the addition of Ru, thus resulting in more negative lattice misfit and more cuboidal γ′ precipitates. The Ru-containing alloy rafts after 2000 h thermal exposure. Meanwhile, TCP phases enriched in Re, W and Cr precipitate after 700 h thermal exposure in the dendrite cores of the Ru-free alloy while TCP phases are not observed even after 2000 h thermal exposure in the Ru-containing alloy. These results suggest that Ru addition suppresses the precipitation of TCP phases significantly. The investigation of stress-rupture properties at 1100 oC /140 MPa indicates that Ru addition increases the stress-rupture life of the baseline alloy (Ru-free) since Ru addition increases the γ′ volume fraction and decreases the width of γ channels after standard heat treatment, as well as increasing the γ/γ′ lattice misfit with a well rafting microstructure.