The effect of withdrawal rates on the solidification microstructure of a single crystal Ni-based superalloy containing ruthenium was investigated. The results show that with the increase of the withdrawal rate, the as-cast microstructure of the single crystal alloy transforms from coarse dendrite to fine dendrite. Meanwhile, the size of the g ￠ particles decreases, segregation of alloying elements for the dendrite and interdendritic regions tends to be reduced, the volume fraction of g-g ￠ eutectic and NiAl-based b phase decreases gradually. In addition, the withdrawal rate has little influence on the phase transformation temperature of the superalloy.