Abstract: The ductile iron planetary carrier used in tractors cracked after approximately 400 hours of use. The fracture reasons of the planetary carrier were analyzed using macroscopic observation, scanning electron microscopy analysis, mechanical property, testing, metallographic examination. The results show that the mechanical properties of the planetary carrier were lower than the standard requirements, and there were serious casting defects such as slag inclusion, shrinkage, and unsoundness, which destroyed the continuity and density of the internal structure of the casting material, reduced the effective bearing area of the casting, lowered the strength of the parts, and caused stress concentration at the location of casting defects, ultimately led to early brittle fracture of the planetary carrier.