The formation mechanism of freckles in large-sized UGTC47 directionally solidified turbine blades for heavy-duty gas turbines and their effects on the stress rupture property performance were investigated using optical microscopy (OM), scanning electron microscopy (SEM), and endurance performance testing. The results indicate that freckle defects form in both the middle and root regions of the blade tenon, with the formation mechanism being the density inversion caused by liquid phase segregation, resulting in fluid convection under the action of gravity, leading to the fracture of dendrite arms and thus the formation of freckles. At 900 ℃/380 MPa, the percentage of freckle area has a significant impact on the endurance performance of the UGTC47 alloy. With the increase in freckle content, the endurance life of the alloy decreases from 131.83 h without freckles to 33.66 h.