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The effects of Hf content on the as-cast microstructures were investigated in DD6 single crystal superalloy by OM, SEM and EDS. The results show that with the Hf content increasing, primary dendrite arm spacing increases first and decreases afterwards, and the volume fraction of γ/γ′ eutectic increases. The size of γ′ phase particles of dendritic core has no obvious change while that of interdendritic decreases a little. Microporosity has no apparent change and extent of elements segregation slightly decreases. There are two kinds of carbides with different color shades in alloys.