A stray grain with the regular shape was found in the transition zone between the airfoil and the platform of some single crystal blades, which are prepared by the Bridgman method. The stray grain is band like and its length direction is parallel or perpendicular to the growth direction of dendrite in the airfoil and the platform. The metallographic method (OM), the electron probe microanalysis (EPMA), and electron backscatter diffraction (EBSD) are used to analyze the microstructure, composition and crystallographic relationship of the stray grain and its surrounding area, respectively. ProCAST is used to simulate the distribution of temperature field and undercooling of platform during solidification. The defect includes many columnar grains, whose composition is not significantly different from the airfoil or platform. However, there exit grain boundary angles between columnar grains and surrounding matrix. The results of temperature distribution show that the undercooling of the transition zone of airfoil and platform is smaller than that of the platform edge. Thus, the solidification speed of the alloy liquid is lower, and the transition zone is the final solidification area. Based on these results, the formation mechanism of defect in the transition zone is proposed, which provides a theoretical basis for eliminating the defects.