The steering output shaft of one vehicle was fractured after use on mountain road for 8 months and 6 days. The accumulating mileage was 35 001 km. The steering gear was analyzed by chemical composition test, macroscopic and microscopic fracture observation, metallographic examination, tensile and mechanical property test, and micro-hardness determination to investigate the fracture cause. The results showed that the fracture of steering output shaft was an one-time overload fracture under the distorting stress. The main reason for the fracture of steering output shaft was the low strength of material after heat treatment and the excessive impact load.