Abstract: The freckle defect formed during directional solidification of single crystal superalloy investment casting cluster were investigated by experiment and theroy analysis. It is found that freckles tend to form at the heating side of the specimens rather than the shadow side. The freckle defects contain a large amount of equiaxed grains with random orientations and the average grain size of freckles is 340 μm. The eutectic content increases gradually from the shadow side to the heating side finally to the the freckle zone of the specimen, which is 1.21%, 2.53% and 5.41%, respectively. Based on the detection and theoretical analysis of freckle defects, it is considered that the unsymmetrical thermal condition will lead to solute enrichment at the heating side of the specimens due to the temperature gradient zone melting(TGZM). The solidification segregation of the dendrites at the heating side is more severe than the shadow side. This phenomenon increases the tendency of convection and flow in the mushy zone of the heating side, and promotes the formation of freckle defects.