Abstract: The small shaft of a 304 stainless steel encoder broke in service. The cause of the encoder small shaft fracture was analyzed by means of macroscopic observation, chemical composition analysis, hardness test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the fracture position of the encoder small shaft was located in the welding heat affected zone at one side of the connecting rod. The carbon element content of the base metal at one side of the encoder connecting rod was relatively high, and there were large number of strip shaped non-metallic inclusions in the structure, resulting in poor mechanical properties. Under the impact of large external forces, the encoder small shaft had brittel fracture.