Abstract: Defects occurred in a planetary gear forging. The causes of defects were analyzed by means of ultrasonic testing, scanning electron microscopy and energy dispersive spectroscopy, and inclusion source analysis. The results show that the unacceptable defects were mainly composed of microcracks and exogenous non-metallic inclusions. The exogenous non-metallic inclusions originated from the corrosion and spalling of the sliding gate during pouring. These inclusions failed to float up completely and aggregated into macro-inclusions during the solidification of molten steel. Optimizing the molten steel refining process, improving the cleanliness of molten steel, and enhancing the quality and stability of refractory materials were the key technical measures to reduce the ultrasonic testing rejection rate and improve the internal quality of steel.