A third-generation single-crystal superalloy WZ30 was used to produce single-crystal samples with varying dendrite spacings under different processing techniques. The microstructures and stress rupture properties were examined and compared. By improving the isolation between heating and cooling zone of the directional solidification furnace, the mean primary dendrite spacing of the sample castings was reduced from 415 μm to 251 μm, leading to a noticeable refinement of dendrite and eutectic structure. At the same time, dendrite refinement can simultaneously decrease the content of casting porosity from 2.29% to 0.21%. Additionally, following dendrite refinement, the γ′ phase in both the dendritic and inter-dendritic regions undergoes refinement, resulting in a more uniform size distribution and a more regular shape. After the subsequent solution and aging treatment, the smaller γ′ precipitates with higher degree of cuboid were obtained in the dendrite-refined samples. As a result, the stress rupture properties at 980 ℃/280 MPa condition was improved by 13.8%.