Abstract: Fracture reasons of motorcycle spindle bars which fractured frequently when straightening after carburizing and quenching heat treatment were analyzed by means of chemical compositions analysis, metallographic examination, hardness test and macro and micro fracture analysis. The results show that a large amount of plastic deformation produced at the spindle bar surface region after extrusion forming which had reached the critical deformation amount. At the same time, normalizing treatment never had been processed to the spindle blank after cold forging according to the technical requirements, which resulted in the original microstructure before heat treatment didn’t meet the requirements. So recrystallization happened at the spindle bar surface region during the carburizing process and resulted in formation of very coarse equiaxed grain region, which formed extremely coarse tempered lath martensite with high brittleness and hardness after quenching and low temperature tempering. So brittle fracture happened to the motorcycle spindles under the action of high external force of straightening.