A failure investigation has been conducted on a transmission gear shaft in a bidirectional gear pump. The fracture failure of studied gear shaft occurred in the running process test on the test bench after it ran about 60 h at the pressure of 21 MPa and the speed of 2000 r/min. A static strength check revealed the gear shaft was safe under normal working conditions. Detailed examinations on the fractured part of this gear shaft indicated that no no-metallic inclusions and other micro-structure defects existence could attribute to the failure of it. Further check on the fracture surface revealed typical tracks formed by fatigue crack propagation. The obvious machining tracks in the arc resulting in stress concentration and exacerbating the initiation and propagation of cracks in these areas.