A single crystal Ni-base superalloy was prepared with five withdrawal rates: 2, 4, 6, 7 and 10 mm/min. Microstructures including dendrite arm spacing, element segregation, and porosity of the as-cast superalloy were investigated. The results showed that as the withdrawal rate increasing, the primary and secondary dendrite arm spacing decreased markedly, the γ/γ′ eutectic became smaller and more dispersive. Meanwhile, when withdrawal rate was higher, W, Ti, Ta and Al segregated in comparatively larger extent. Furthermore, as the withdrawal rate increasing, the amount of alloy microporosity increased, though the size of which decreased gradually. It can be concluded that the withdrawal rate of 4 mm/min and 6 mm/min are optimal for the experimental alloy.