The as-cast microstructure, the heat-treated microstructure and the composition characteristics of freckle chains in the third generation single crystal superalloy DD9 were studied by OM, SEM, EBSD and TEM. The results show that freckles are composed of fine disorder-oriented equiaxed grains. They are in the shape of a chain roughly along the direction of crystal growth and only appear on the surface of single crystal castings. The depth and width of freckle chains are 400~800 μm. Freckled area contains a large amount of γ/γ' eutectics. A certain amount of MC carbides and porosity also appear in as-cast freckled area. After heat treatment, large-size γ' phase and granular M₆C phase appear at the equiaxed grain boundary of freckled region. The freckled area contains more Ta and Al, but less Re and W. The composition of the freckled area is similar to the interdendritic composition.