An unqualified six-cylinder heavy truck crankshaft has been studied to investigate the cause of magnetic particle testing defects on the rod joals. Large-sized long-striped MnS inclusions are regarded as the major cause for the magnetic parti-cle testing failure because they have been detected in situ under the magnetic particle indications. Through the observation of macroscopic structures of the rod joals and corresponding counterweight blocks, it is found that for the 1# and 3# rod joals, the center metal of the original hot-rolled bar has been extruded to the inboard edge of the rod joals and large-sized long-striped MnS inclusions are exposed on the surface after flash removal, leading to the failure of magnetic particle testing. As for the 2# rod joal, the center metal of the original bar has not been extruded to the surface and MnS inclusions on the rod joal surface are small in size, few in number, resulting in passing the magnetic particle testing. If the quality of the hot-rolled bars fluctuates, it is more recommended to apply magnetic particle testing on samples at the center of bars before forging to evaluate the severity of defects caused by the long-striped MnS inclusions for fear of the scrap of the final crankshafts.