Abstract: The fracture of chain wheel drive shaft in an oil drilling drawwork during operation ocurred. The reason for fracture of the chain wheel drive shaft was studied by fracture morphology observation, chemical composition analysis, mechanical property test and microstructure observation. The results show that the fracture of chain wheel drive shaft was early fatigue failure. The cracks initiated at the stress concentration of the rough transition fillet root at the step of φ210 mm and φ200 mm shafts of the chain wheel drive shaft, and expanded under alternating bending and torsion stresses, leading to the fatigue fracture of the chain wheel drive shaft finally. The existence of lamellar MnS inclusions in the structure and the banded segregation in the core promoted the growth of fatigue cracks.