Abstract: The 35 steel pin for an oscillating arm oil cylinder of one type milling machine fractured after service for around 2 a. The reason for the pin fracture was studied by the methods such as chemical composition analysis, hardness test, microstructure and surface topography observation, finite element simulation and fracture analysis. The results show that the batch of pins had a multi-source fatigue fracture. The fracture surface was smooth and the crack source was located in the root of the turning tool mark on pin surface. The main reason for the pin fracture was that the machining surface of the pin was rough and the stresses were concentrated in the root of tool mark, leading to the formation of crack source in the tool mark root of the central pin under the working load. Meanwhile the pin was not subjected to quenching and tempering as required, resulting in the coarse microstructure and the easy propagation of fatigue cracks.