Small fatigue crack behaviors in a nickel-based directionally solidified superalloy DZ4 were studied by in situ scanning electron microscopy. The crack initiation and propagation manners were identified under different temperatures, i.e. 25oC, 350oC, 700oC. Crack growth showed to be associated with the configuration and relative direction of carbide. The anomalous short crack growth was pronounced at 25oC, 350oC and not evident at 700oC, which was analyzed by in situ examining the effect of microstructure.