Abstract: Abstract A certain planetary gear experienced cracking and tooth breakage after long-term service. The reasons for gear tooth breakage were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination, hardness testing, and depth testing of the carburized layer. The results show that there were non-metallic inclusions mainly composed of Al₂O₃ in the first gear, which caused stress concentration and fatigue crack initiation and propagation under cyclic loading, ultimately led to fatigue cracking of the first gear. The cracking of gear 1 reduced the effective load-bearing area of the component and increased stress, and subsequently led to overload and tooth breakage accidents of gears 2 and 3.